Mix the dough (morning): In a large bowl, combine 100 g active starter, 375 g water, and 500 g bread flour. Stir until no dry bits remain. Rest 20 minutes (autolyse), then add 10 g fine sea salt and 20 g olive oil. Squeeze and fold until mostly smooth.

Bulk ferment: Cover and rest at room temperature for 3–4 hours. During the first 90 minutes, do 3 sets of stretch-and-folds every 30 minutes to build strength. The dough should look puffy and elastic.

Pan and proof: Generously oil a 9x13-inch pan (or a half sheet pan for thinner focaccia). Gently turn the dough into the pan and flip to coat in oil. Rest 20 minutes, then coax it toward the corners. Cover and proof until noticeably airy and wobbly, 60–90 minutes.

Prep the topping: Toss 1–1.5 cups blueberries with 1–2 tablespoons sugar and the zest of 1 lemon. If you like, add 1/2 teaspoon vanilla. Set aside.

Dimple and top: Heat oven to 425°F (220°C). Oil your fingers and press straight down to dimple the dough all over, creating little wells. Scatter blueberries evenly, pressing some lightly into the dimples. Drizzle 1–2 tablespoons olive oil over the top and add a pinch of flaky salt if using.

Bake: Bake 22–28 minutes, until edges are deep golden and the top is browned with bubbling juices. If berries look pale, give it another 2–3 minutes. The bottom should be crisp.

Finish: Cool in the pan 10 minutes, then loosen edges and transfer to a rack. While warm, you can brush with a thin glaze of honey and lemon juice for shine. Slice once it’s warm but no longer fragile.