In a large bowl, whisk together the flour, kosher salt, and instant yeast.
4 cups all-purpose flour, or bread flour, 2 tsp kosher salt, 2 tsp instant yeast
Add in the lukewarm water.
2 cups lukewarm water
Using a rubber spatula, mix until the liquid is absorbed and the ingredients form a sticky dough ball.
Rub the surface of the dough lightly with olive oil.
4 tbsp olive oil
Cover the bowl with a damp tea towel, cloth bowl cover, or plastic wrap.
Refrigerate for 12 - 36 hours* (see notes).
Grease two 9x13 pans with butter and olive oil. This is an important step to keep the bread from sticking to the pan.
1 tbsp butter
Pour some more olive oil into each casserole dish. Add an equal size dough portion to each dish.
Allow the dough to rise for 3-4 hours.
It is not necessary to cover the dough while it rises, but I do just because.
After rising, the dough should mostly fill each casserole dish.
Set a rack in the middle of the oven and preheat to 425℉. Pour two tablespoons of olive oil over each dough round. Coat your hands in olive oil. Use all of your fingers to press straight down into the dough to create deep dimples. Gently stretch the dough so it fills the pan.
Generously sprinkle with flaky sea salt and rosemary leaves.
2 tsp rosemary leaves, 1 tbsp flaky sea salt
Add the dough to the pre-heated oven. Bake for 25 to 30 minutes, or until it is golden and crisp all over (including the bottom).
Remove the bread from the oven, and transfer to a rack to cool.