Earthy Sourdough Focaccia

1. Mix the dough. In a large bowl, whisk together 500 g bread flour and 10 g fine sea salt. Add 375 g water and 100 g active starter. Mix with a wet hand or spatula until no dry flour remains. Drizzle in 25 g olive oil and pinch and squish until it is absorbed. It will feel slippery at first, then suddenly cohesive and glossy. Cover and rest 20 minutes.
2. Build strength with stretch-and-folds. Over the next 60 to 90 minutes, do 3 to 4 rounds of stretch-and-folds (every 20 to 30 minutes). Cover between rounds. The dough should go from shaggy to smoother and bouncier.
3. Bulk ferment. Let the dough rise at room temperature until it looks puffy and has increased by about 50 to 75 percent, typically 3 to 5 hours depending on kitchen temperature and starter strength.
4. Cold ferment (recommended). Cover tightly and refrigerate 8 to 12 hours for deeper flavor and easier handling. If you are baking same day, you can skip this and move on once the dough is nicely airy.
5. Cook the mushroom topping. Heat 2 tbsp olive oil in a skillet over medium-high heat. Add mushrooms and cook until browned and most moisture cooks off, 6 to 10 minutes. Stir in garlic, thyme, rosemary, salt, and pepper for 30 to 60 seconds. Turn off heat. Cool to room temp.
6. Oil the pan generously. Pour 3 to 4 tbsp olive oil into a 9x13-inch metal pan. Tilt to coat the bottom and corners. Transfer dough into the pan. Flip once so both sides get lightly oiled, or just turn it to coat as best you can. Do not stress. Cover and rest 1 hour at room temperature.
7. Stretch to fit. With oiled fingertips, gently stretch the dough toward the edges. If it resists, wait 10 minutes and try again. Do not force it. Cover and let rise until very puffy and jiggly, 1 to 2 hours.
8. Preheat. Heat oven to 450°F. If you have a baking stone or steel, place it in the oven to preheat too.
9. Dimple and top. Oil your fingertips. Press straight down to make deep dimples across the dough. Spoon the cooled mushroom mixture evenly over the top. Sprinkle Parmesan if using. Finish with a pinch of flaky salt.
10. Bake. Bake on the middle rack (or on the preheated stone or steel) until deeply golden with crisp edges, 20 to 28 minutes. Doneness cues: the edges pull slightly from the pan, the bottom is deep golden, and the center reads about 205 to 210°F if you like to temp bread. If your top browns too fast, loosely tent with foil for the last few minutes. For a half-sheet pan, start checking earlier, around 18 to 22 minutes.
11. Cool, then slice. Let cool in the pan 10 minutes, then lift out and cool on a rack another 10 minutes. Slice and serve warm. If you are dipping in olive oil, you are making excellent choices.

My tiny trick: If you want extra crunch, slide the focaccia out of the pan and bake it directly on the oven rack for 2 to 3 minutes at the end. Keep an eye on it.