In the morning--around the time you'd usually feed your sourdough starter--add all-purpose flour, bread flour, 344g water (reserve remaining 50g for later), salt and 94g mature sourdough starter to a large bowl. Mix by hand or in the bowl of a stand mixer with the dough hook attachment on low speed just until the mixture forms a dough. Continue to knead the dough by hand, or in a stand mixer on medium speed, until the dough becomes smooth and elastic (about 5 minutes by hand, 3 to 4 minutes in a stand mixer).

Slowly add remaining 50g water to dough, kneading or mixing on low speed as you go. Continue to knead or mix on medium speed until mixture re-forms into a soft, smooth mass of dough. Drizzle in olive oil, kneading or mixing on low speed as you go. Mix the dough and oil together until everything is fully combined and the dough is smooth and elastic (about 3 to 5 minutes by hand, 1 to 2 minutes in a stand mixer).

Use a silicone spatula or a bowl scraper to scrape dough into a clean, large bowl oiled with olive oil. Cover bowl with plastic wrap and let rest in a warm place for 30 minutes (I placed my bowl in the oven, turned off, with the oven light on).

After 30 minutes have elapsed, uncover bowl. Perform one set of stretch and folds by grabbing one side of the dough, stretching it up and flopping it over on itself. Turn the bowl 180 degrees and repeat, then rotate 90 degrees and stretch and fold, rotate 180 degrees and stretch and fold the fourth side. Cover the bowl again and return it to its resting place. Repeat this step (one set of stretch and folds) every 30 minutes over the course of 2 hours. You will end up giving the dough 4 sets of stretches and folds total.

Spread 2-3 tablespoons of olive oil into a 13x9-inch baking pan. After the 2 hours have elapsed, transfer the dough gently (so as not to disturb any bubbles that may have formed) to the prepared baking pan. Cover the pan with plastic wrap and let the dough rest 2 hours, uncovering every 30 minutes to gently stretch the dough with oiled hands towards the corners of the pan (it will take the entire 2 hours for the dough to relax and stretch, but don't worry--it will get there!).

After these 2 hours have elapsed, leave the dough covered and untouched for another 2 hours in its warm place. This is when the dough will puff up and develop more bubbles. About 30 minutes before you plan to bake the bread, preheat the oven to 400°F (if you have the dough proofing in the oven, TAKE IT OUT before preheating!).

Drizzle about 1-2 tablespoons olive oil evenly on top of dough, then use oiled fingers to dimple the top of the dough all over, pressing all the way to the bottom of the pan. Sprinkle with desired toppings.

Bake focaccia in preheated oven until golden brown on top, about 30 minutes, rotating pan halfway through baking. Cool 5 minutes in baking pan, then carefully transfer to a cooling rack. Serve warm or at room temperature.

Store bread in an airtight container at room temperature up to 3 days.