Introduction
Picture this: it’s a chilly evening, maybe rain is tapping against the window, and you’re craving something warm, savory, and utterly comforting. Something that feels a little fancy but is surprisingly easy to whip up. That, my friend, is where this French Onion and Mushroom Tart with Gruyere cheese swoops in! It’s like a hug on a plate, combining the deep, sweet flavor of slow-caramelized onions with earthy mushrooms and nutty, melted Gruyere, all nestled in a flaky pastry crust. Honestly, the smell alone while it’s baking is enough to make your heart happy. This recipe is a classic for a reason – it’s simple, uses everyday ingredients, and delivers maximum flavor with minimal fuss. It’s the kind of dish that makes weeknight dinners feel special or elevates a weekend brunch without stress. Trust me, once you make this, you’ll keep coming back to it!
Why You’ll Love This Recipe
- Fast(ish): While the onions take a little love (totally passive cooking time, though!), the assembly is super quick.
- Easy: No complicated techniques here! Just simple steps for big flavor.
- Giftable/Shareable: It travels well and is always a welcome addition to a potluck or a thoughtful meal for a friend.
- Crowd-Pleasing: Seriously, who can resist flaky pastry, savory filling, and gooey cheese? Kids and adults alike adore this.
Ingredients
Gather your goodies! Here’s what you’ll need:
- 1 sheet (approx. 320g) ready-rolled puff pastry or shortcrust pastry: Use your favorite! Puff pastry gives you that glorious flaky lift, while shortcrust is classic and sturdy. Whatever you choose, using ready-made is our little secret weapon for keeping this easy.
- 3 large onions, thinly sliced: The star of the show! Thin slices are key for even caramelization. Don’t rush this step!
- 300g mushrooms, sliced: Cremini (baby bellas) or white button mushrooms work perfectly.
- 100g Gruyere cheese, grated: Oh, Gruyere! Its nutty, slightly sweet flavor is divine here.
- 2 large eggs: The binding magic for our creamy custard base.
- 150ml double cream or heavy cream: Adds richness and that luscious texture to the filling.
- 50ml milk: Thins out the custard just right. Whole milk is great if you have it.
- 1 tbsp butter: Helps those onions caramelize beautifully.
- 1 tbsp olive oil: Also aids in the onion cooking process and adds a little flavor.
- 1 tsp fresh thyme leaves (optional): Fresh thyme is chef’s kiss with onions and mushrooms, but if you don’t have it, don’t sweat it.
- Salt, to taste: Essential for bringing out all the flavors.
- Black pepper, to taste: A little kick to balance the richness.
How to Make It
Let’s get baking! Don’t be intimidated; this is straightforward, I promise.
- Prep Your Oven & Pastry: Preheat your oven to 190°C (170°C fan/Gas Mark 5). Grab your favorite 23cm tart tin. If you’re using puff pastry, just unroll it and gently press it into the tin, trimming off any big excess. For shortcrust, you’ll roll it out a bit first, then line the tin. Give the base a little prick all over with a fork – this helps prevent it puffing up too much. Depending on your pastry package instructions, you might need to do a quick blind bake here (lining it with parchment and filling with baking beans/rice for a bit, then removing them). I sometimes skip the blind bake with good quality puff pastry if I’m feeling lazy, but follow the package if it recommends it!
- Caramelize Those Onions: This is the most important step for flavor! Heat the butter and olive oil in a large pan over medium-low heat. Toss in your thinly sliced onions. Now, this is where patience comes in. Cook them slowly for 20-30 minutes, stirring occasionally. You want them to soften, turn sweet, and develop a deep golden-brown color. Don’t rush this part – it’s worth the wait!
- Sauté the Mushrooms: While the onions are doing their thing in the background, get a separate pan going over medium-high heat. Add your sliced mushrooms and cook, stirring. They’ll release their liquid first, and then start to brown up nicely, about 5-7 minutes. You want them cooked down a bit.
- Combine the Filling: In a bowl, mix together your gorgeous caramelized onions, the cooked mushrooms, the grated Gruyere cheese, and those fresh thyme leaves if you’re using them. Give it a good season with salt and pepper. Taste a little bit! Does it need more salt?
- Fill the Pastry: Spread this yummy onion and mushroom mixture evenly over the base of your prepared pastry shell in the tart tin.
- Whip Up the Custard: Grab a jug (makes pouring easy!) and whisk together the eggs, cream, milk, and another little pinch of salt and pepper. Get it all nice and combined.
- Pour and Bake: Carefully pour the egg mixture all over the filling in the tart shell. Make sure it gets into all the little nooks and crannies. Pop the tart into your preheated oven. Bake for 25-35 minutes. You’ll know it’s done when the filling is set (it won’t jiggle too much in the center) and the top is beautifully golden brown.
- Cool Slightly: Once it’s out of the oven, resist the urge to slice immediately! Let the tart cool in the tin for 5-10 minutes. This helps it set up properly so it doesn’t fall apart when you cut into it.
- Serve and Enjoy! Slice it up and serve warm. It’s perfect on its own or with a simple green salad.
Substitutions & Additions
Feel free to play around and make this tart your own!
- Cheese Swap: No Gruyere? No problem! Sharp cheddar, Swiss, or even fontina would be delicious substitutes. A mix of cheeses is great too!
- Add Veggies: A handful of spinach (wilt it with the mushrooms), some roasted bell pepper strips, or even some asparagus spears would be lovely additions.
- Make it Meaty: A little cooked crumbled bacon or some diced ham would add a savory depth. Add it in with the onion/mushroom mixture.
- Different Herbs: Swap thyme for fresh rosemary or chives.
- Pastry: You could even make your own pastry if you’re feeling ambitious!
Tips for Success
Here are a few little pointers I’ve picked up along the way to make sure your tart turns out perfectly:
- Slow & Low for Onions: I cannot stress this enough! Low heat and patience are key to sweet, caramelized onions, not burnt ones.
- Don’t Crowd Mushrooms: Cook mushrooms in batches if your pan isn’t huge, so they brown instead of steam.
- Check Pastry Package: Pay attention to whether your specific pastry needs blind baking. It can make a difference in preventing a soggy bottom.
- Don’t Overfill: Make sure the egg mixture doesn’t go right to the very top edge, as it will puff slightly.
- Rest is Best: Letting it cool briefly before slicing helps the custard set up.
How to Store It
If you have any leftovers (a big IF!), store them in an airtight container in the refrigerator for up to 3-4 days. To reheat, pop a slice in the oven at 160°C (325°F) for about 10-15 minutes, or until warmed through. You can microwave it, but the pastry won’t be as crisp.
FAQs
Got questions? Let’s tackle a few common ones!
Can I make the filling ahead of time?
Yes! You can caramelize the onions and cook the mushrooms a day in advance. Store them covered in the fridge. When you’re ready to assemble, just warm them slightly if they’re stiff, then proceed with the recipe.
Can I use frozen pre-sliced onions?
While fresh onions are always best for caramelizing, you can use frozen if you’re in a pinch. Thaw them first and make sure to cook out all the excess liquid before trying to brown them.
Is this tart good cold?
Absolutely! It’s delicious warm, but leftovers are fantastic cold straight from the fridge too, making them great for lunchboxes or picnics.
Can I freeze this tart?
Tarts with creamy fillings can sometimes change texture a bit when frozen and reheated. If you want to try, bake the tart fully, let it cool completely, then wrap it well before freezing. Thaw in the fridge overnight and reheat gently in the oven. The pastry might lose some crispness.
Cozy French Onion & Mushroom Gruyere Tart
A warm, savory, and utterly comforting tart combining the deep, sweet flavor of slow-caramelized onions with earthy mushrooms and nutty, melted Gruyere, all nestled in a flaky pastry crust. It's surprisingly easy to whip up and perfect for weeknight dinners or weekend brunch.
Equipment
- Oven
- tart tin 23cm
- large pan
- Pan
- bowl
- jug
- Fork
- Whisk
Ingredients
Main Ingredients
- 1 sheet ready-rolled pastry puff pastry or shortcrust pastry, approx. 320g
- 3 large onions thinly sliced
- 300 g mushrooms sliced (cremini or white button)
- 100 g Gruyere cheese grated
- 2 large eggs
- 150 ml double cream or heavy cream
- 50 ml milk whole milk is great
- 1 tbsp butter
- 1 tbsp olive oil
- 1 tsp fresh thyme leaves optional
- Salt to taste
- Black pepper to taste
Instructions
- Prep Your Oven & Pastry: Preheat your oven to 190°C (170°C fan/Gas Mark 5). Grab your favorite 23cm tart tin. If you're using puff pastry, just unroll it and gently press it into the tin, trimming off any big excess. For shortcrust, you'll roll it out a bit first, then line the tin. Give the base a little prick all over with a fork – this helps prevent it puffing up too much. Depending on your pastry package instructions, you might need to do a quick blind bake here (lining it with parchment and filling with baking beans/rice for a bit, then removing them). I sometimes skip the blind bake with good quality puff pastry if I'm feeling lazy, but follow the package if it recommends it!
- Caramelize Those Onions: This is the most important step for flavor! Heat the butter and olive oil in a large pan over medium-low heat. Toss in your thinly sliced onions. Now, this is where patience comes in. Cook them slowly for 20-30 minutes, stirring occasionally. You want them to soften, turn sweet, and develop a deep golden-brown color. Don't rush this part – it's worth the wait!
- Sauté the Mushrooms: While the onions are doing their thing in the background, get a separate pan going over medium-high heat. Add your sliced mushrooms and cook, stirring. They'll release their liquid first, and then start to brown up nicely, about 5-7 minutes. You want them cooked down a bit.
- Combine the Filling: In a bowl, mix together your gorgeous caramelized onions, the cooked mushrooms, the grated Gruyere cheese, and those fresh thyme leaves if you're using them. Give it a good season with salt and pepper. Taste a little bit! Does it need more salt?
- Fill the Pastry: Spread this yummy onion and mushroom mixture evenly over the base of your prepared pastry shell in the tart tin.
- Whip Up the Custard: Grab a jug (makes pouring easy!) and whisk together the eggs, cream, milk, and another little pinch of salt and pepper. Get it all nice and combined.
- Pour and Bake: Carefully pour the egg mixture all over the filling in the tart shell. Make sure it gets into all the little nooks and crannies. Pop the tart into your preheated oven. Bake for 25-35 minutes. You'll know it's done when the filling is set (it won't jiggle too much in the center) and the top is beautifully golden brown.
- Cool Slightly: Once it's out of the oven, resist the urge to slice immediately! Let the tart cool in the tin for 5-10 minutes. This helps it set up properly so it doesn't fall apart when you cut into it.
- Serve and Enjoy! Slice it up and serve warm. It's perfect on its own or with a simple green salad.
Notes
Tips for Success: Slow & Low heat is key for sweet, caramelized onions. Don't crowd mushrooms in the pan so they brown instead of steam. Check your pastry package instructions for recommended blind baking. Don't overfill the tart with the egg mixture. Letting the tart cool briefly before slicing helps the custard set up.
Storage: Store leftovers in an airtight container in the refrigerator for up to 3-4 days. Reheat slices in the oven at 160°C (325°F) for 10-15 minutes, or until warmed. Microwaving is possible but pastry won't be as crisp.
Make Ahead: The caramelized onions and cooked mushrooms can be made a day in advance and stored covered in the fridge. Warm slightly before assembling.
Freezing: Tarts with creamy fillings can change texture when frozen. If freezing, bake fully, cool completely, wrap well, and freeze. Thaw in the fridge overnight and reheat gently in the oven. The pastry may lose some crispness.
Substitutions: Sharp cheddar, Swiss, or fontina can substitute Gruyere. Add veggies like spinach, roasted bell pepper, or asparagus. Add cooked crumbled bacon or diced ham. Swap thyme for fresh rosemary or chives.