In a large mixing bowl, stir together the starter, kosher salt, and water with a spatula until combined. It does not need to be smooth. Add the bread flour and stir until no dry flour remains.

About 30 minutes after mixing, perform one set of stretch-and-folds: grab the dough from one side, pull it up and over to the center, rotate the bowl a quarter turn, and repeat 8 to 10 times.

Drizzle a little olive oil over the dough, rub the surface to coat, and cover the bowl with a tea towel or bowl cover. Let it rise at room temperature, around 70F, for 4 to 18 hours, until the dough has nearly doubled in volume. In a warm summer kitchen this may take 4 to 6 hours; in winter it can take 12 to 18 hours. Rely on visual cues rather than the clock, and avoid letting it rise in an oven with the light on because it will be too warm.

When the dough has doubled, pour 2 tablespoons of the olive oil into a 9 by 13-inch baking pan and tilt to coat the bottom. If using a glass pan, butter it first to avoid sticking.

Drizzle the dough with the remaining 1 tablespoon olive oil. With lightly oiled hands, gently deflate the dough and release it from the sides of the bowl. Transfer it to the center of the prepared pan, then fold it envelope-style from top to bottom and side to side to form a rough rectangle. Flip the dough so the seam is facing down.

Rub the top of the dough with a little more olive oil and leave it uncovered for 4 to 6 hours, until puffy and almost doubled in size.

Heat the oven to 425F. Lightly oil your hands, then press your fingertips firmly into the dough to create deep dimples and stretch it toward the edges of the pan. Sprinkle generously with flaky sea salt. Bake for 25 minutes, or until deep golden all over. Transfer the focaccia to a wire rack and cool for at least 20 minutes before slicing.