1. Combine the Wet Ingredients and Starter: In a large mixing bowl, gently whisk together the active sourdough starter, warm water, olive oil, and vanilla extract until well combined and the starter is mostly dissolved. The mixture should look milky and slightly frothy.

2. Mix the Dry Ingredients: In a separate bowl, combine the all-purpose flour, granulated sugar, and fine sea salt. Whisk them together to ensure they are evenly distributed.

3. Form the Dough: Add the dry ingredients to the wet ingredients. Using a sturdy spoon or your hands, mix until a shaggy dough forms and no dry streaks of flour remain. It will be sticky at this stage. Cover the bowl with a clean kitchen towel or plastic wrap and let it rest for 30 minutes. This allows the flour to fully hydrate.

4. Perform Stretch and Folds: After the rest, perform a series of “stretch and folds.” Lightly wet your hands, grab a portion of the dough from one side, stretch it upwards, and fold it over to the opposite side of the bowl. Rotate the bowl a quarter turn and repeat this process 3-4 more times until you’ve worked your way around the dough. Repeat this stretch and fold process every 30 minutes for 2-3 hours. This builds strength in the dough.

5. Bulk Fermentation: After the stretch and folds, cover the bowl again and let the dough bulk ferment at room temperature for 1-2 hours, or until it has visibly increased in volume by about 20-30%. For a deeper flavor, you can place the covered bowl in the refrigerator for 8-12 hours (cold fermentation).

6. Prepare for Baking and Proofing: Lightly grease a 9×13 inch baking pan with olive oil. Gently transfer the dough into the prepared pan. Drizzle a little more olive oil over the top of the dough. Using your fingertips, gently dimple the dough all over, creating small indentations. Arrange the sliced strawberries evenly over the top of the dough, pressing them lightly into the dimples. Sprinkle generously with extra granulated sugar.

7. Final Proof: Cover the pan loosely with plastic wrap or a kitchen towel and let it proof at room temperature for 1-2 hours, or until the focaccia looks puffy and has increased in size. Towards the end of the proofing time, preheat your oven to 400 F (200 C).

8. Bake the Focaccia: Bake for 25-30 minutes, or until the focaccia is golden brown around the edges and the strawberries are soft and slightly caramelized. Remove from the oven and let it cool in the pan for a few minutes before transferring to a wire rack to cool completely.