Preheat your oven to 400°F (200°C). If you have a large baking sheet, line it with parchment paper for super easy cleanup.
In a large bowl, toss together the diced sweet potatoes, halved Brussels sprouts, and sliced red onion. Drizzle with the olive oil, add the minced garlic, Italian seasoning (or Herbes de Provence), salt, and pepper. Toss everything gently until the veggies are evenly coated.
In a small bowl, whisk together the maple syrup and Dijon mustard until smooth. This is your glorious glaze!
Add the sliced smoked sausage to the bowl with the seasoned vegetables. Pour the maple-dijon glaze over everything. Toss gently again until the sausage and veggies are well coated in that sticky-sweet-tangy sauce.
Spread the mixture out in a single layer on your prepared baking sheet. Make sure not to overcrowd the pan – the veggies need space to roast and caramelize, not steam. If your pan is too small, use two!
Pop the sheet pan into your preheated oven. Roast for 20-25 minutes, or until the sweet potatoes are tender, the Brussels sprouts have some lovely crispy edges, and the sausage is heated through and maybe a little caramelized. About halfway through (around 10-12 minutes), give everything a good stir or flip to ensure even cooking and browning.
Once everything is beautifully roasted and tender-crisp, carefully remove the sheet pan from the oven.
Serve immediately! You can garnish with fresh chopped parsley and parmesan shavings if you like. Enjoy that incredible flavor combo!