Preparation

• In a large bowl, stir together the water and active starter (test to make sure a little of the starter floats before adding it all). Add half the flour, and mix well by hand, (a few lumps are okay). Cover, and let stand at room temperature for 30 minutes. (This stage is called ”autolyze”; it begins the hydration and gluten development.)
• Add the remaining flour and kosher salt, and mix by hand until uniform; it will be wet and shaggy. Cover and rest at room temperature for 1 hour.
• Fold the ball of dough over on itself 4 times, rotating the bowl as you go (I like to use a bowl scraper to do this). Cover, and let rest 1 hour. Do this two more times, for a total of 3 folds in 3 hours. Cover, and refrigerate for about 10 hours. (This stage is called bulk fermentation.)
• Remove the dough from the refrigerator. Oil a 9×13-inch baking pan or 15×10-inch pan with 2 Tbs. of the oil. Gently transfer the dough to the pan using a bowl scraper, fold it over itself, then gently press it into a rectangle. It doesn’t need to fill the pan completely as it will spread as it rests. Cover with a clean dish towel, and let rise at room temperature for about 1 hour.
• Position a rack in the center of the oven, and preheat the oven to 450°F. Remove the towel; the dough should have bubbles on the surface. Gently use your fingers to dimple the dough, gently push it into the corners of the pan, if needed. (The oil should pool nicely in the corners.) Drizzle the remaining 1 Tbs. oil over the dimples in the dough. Gently press in any toppings onto the dough (resist the urge to cover it like a pizza, as you want about 50% of the dough to be visible). Cover with a clean dish towel, and let rest 20 minutes.
• Remove the towel, and sprinkle with flaky sea salt. Bake until golden and bubbly with a few charred bubbles, about 30 minutes. Use tongs or a spatula to carefully transfer the focaccia to a wire rack to cool (this will ensure a crisp bottom).