Mix starter and flour:

Combine the starter, salt, and water in a large bowl using a spatula; uniform mixing is not required. Add the flour and stir until fully incorporated.

Perform optional fold:

Optionally, perform a single fold 30 minutes after mixing: reach into the bowl, pull the dough upward and toward the center, rotate the bowl a quarter turn, and repeat 8 to 10 times.

Bulk ferment dough:

Drizzle a small amount of olive oil over the dough and rub to coat. Cover with a tea towel or bowl cover and let rise at room temperature (70°F (20°C)/21°C) for 4 to 18 hours. Rise time depends on season, starter strength, and kitchen temperature—e.g., in summer it may double in 6 hours, in winter in 18 hours. Do not use an oven with the light on as it will be too warm. Judge doneness by volume doubling rather than time; a straight-sided container helps monitor this.

Oil the baking pan:

Once the dough has doubled, pour 2 tablespoons of olive oil into a 9x13-inch pan. (A USA Pan works well without sticking; for glass pans, buttering first is recommended to prevent sticking.)

Shape dough in pan:

Drizzle the dough with 1 tablespoon of olive oil. Gently deflate the dough with your hand, releasing it from the bowl sides. Scoop the dough into the center of the oil in the pan. Fold envelope-style from top to bottom and side to side to form a rough rectangle, then flip seam-side down.

Second rise until puffy:

Rub the top of the dough with oil. Leave uncovered for 4 to 6 hours until puffy and nearly doubled in size.

Dimple, salt, and bake:

Preheat the oven to 425°F (220°C). Lightly oil your hands, then use all ten fingers to press dimples into the dough, stretching it to nearly fill the pan. Sprinkle generously with flaky sea salt. Bake for about 25 minutes until golden all over. Transfer the bread to a cooling rack and cool for at least 20 minutes before slicing.