Prepare the Sourdough Starter Base: In a small bowl, mix together 5g ripe sourdough starter, 55g room temperature water, and 55g all-purpose or bread flour. Let sit at room temperature for 4-8 hours until bubbly and tangy.

Roast the Garlic: Preheat oven to 400°F (200°C). Slice the tops off 3 bulbs of garlic, drizzle with olive oil, wrap in foil, and roast on a baking sheet for 40-50 minutes until soft and golden brown. Let cool completely.

Make the Focaccia Dough: In a large bowl, combine 100g active sourdough levain, 400g room temperature water, and 500g bread flour. Mix until shaggy. Add 13g salt, the mashed flesh of 2 cooled roasted garlic bulbs, and 3.5g finely chopped rosemary. Knead for 5-10 minutes until smooth and elastic.

First Fermentation: Cover the bowl and let the dough rise at room temperature for 4-6 hours, or until doubled in size. Perform gentle stretch and folds every 30-60 minutes during the first couple of hours.

Shape and Second Fermentation: Generously oil a 9x13 inch pan. Gently transfer the dough to the pan, drizzle with more olive oil, and use oiled fingertips to stretch and dimple the dough to fill the pan. Cover and let rest at room temperature for 1-2 hours until puffy.

Prepare the Topping: In a small bowl, whisk together 70g olive oil, flaky sea salt, the mashed flesh of the remaining roasted garlic bulb, and 1 sprig of finely chopped rosemary.

Bake the Focaccia: Preheat oven to 425°F (220°C). Dimple the surface of the focaccia with oiled fingertips. Drizzle the prepared topping evenly over the dough and sprinkle with extra flaky sea salt if desired. Bake for 20-25 minutes until golden brown.

Cool and Serve: Let the focaccia cool in the pan for 10-15 minutes, then transfer to a wire rack to cool completely before slicing and serving.