Sourdough Focaccia

Levain: On day 1 at night, mix together starter, flour, and water in a large bowl. Cover with a damp kitchen towel or a large plate and let stand at room temperature 10 to 12 hours.

Dough: On day 2 in the morning, make an autolyze: In a large bowl, dissolve levain in water. Add both flours and stir to combine. Cover with a damp kitchen towel and let stand at room temperature 1 hour.

Sprinkle autolyze with salt; drizzle with 1 tablespoon oil. Pinch and fold several times to incorporate. Cover and let stand 30 minutes.

With dampened hands, grab underside of dough, stretch it out, and fold it back over itself. Rotate bowl one quarter-turn and repeat 6 times. Cover and let stand 30 minutes. Repeat process 4 more times (about 3 hours total). After final folding, you should have a soft, elastic dough.

Drizzle remaining 3 tablespoons oil evenly into a 9 1/2-by-12 1/2-inch rimmed baking sheet or a 9-by-12-inch baking pan. Scrape dough onto sheet and pat out to flatten, pushing it into edges. (If it springs back, cover and let stand 10 minutes, then continue until dough fills pan.) Drape with plastic wrap. (Dough can be made to this point and refrigerated up to 24 hours.)

Let dough stand until it is bubbly and soft and almost reaches top of pan, about 2 hours (or 3, if it's been refrigerated). Preheat oven to 450°F. Use your fingers to make dents all over top of dough. Drizzle generously with oil and sprinkle with flaky salt. Season tomatoes with fine salt and scatter over dough. Sprinkle with rosemary.

Place a foil-lined rimmed baking sheet on lowest rack (to catch any oil overflow). Bake focaccia on center rack until golden brown and cooked through, 25 to 30 minutes. Let cool on sheet on a wire rack 10 minutes, then transfer directly to rack and let cool completely. (You may need to run a spatula underneath to release it.) Focaccia is best served the day it's made, but can be stored in an airtight container at room temperature up to 2 days.