In a large bowl, whisk together sourdough starter, warm water, olive oil, and salt. Add flour and mix until no dry flour remains and a shaggy dough forms. It will be a wet and stick dough but will smooth out as we do the stretch and folds.

Set the dough aside for 30 minutes to rest.

Generously oil a cookie sheet with olive oil. Transfer the dough to the baking dish and preform the first set of stretch and folds. Starting with the back edge of the dough, pull the dough up and out of the pan by about a food. Fold the dough over and rotate the pan 1/4 turn. Stretch and fold the dough 3 more times, up and over. The dough should become smooth and supple. Flip the dough over, seam side down on the cookie sheet, cover with plastic and let rest 30 minutes.

Perform 3 more sets of stretch and folds, 30 minutes apart. After the final stretch and fold, cover the dough and allow to rise at room temperature for 6-8 hours, until puffy and roughly doubled. The total time for the bulk rise from the time of mixing to shaping should be around 8-10 hours, depending on the temperature of your kitchen.

Once doubled, spread about 2 tbsp of olive oil over the top of the focaccia and dimple the surface with your fingertips by pressing them into the dough towards the bottom of the dish, working from the center out to spread the dough towards the edges of the dish. The dough should cover about 3/4 of the pan.

Cover with plastic wrap and place the baking dish in the refrigerator for at least 8 hours but up to 5 days.

When ready to bake, preheat oven to 450°F. Drizzle dough again lightly with olive oil and dimple with fingertips, pushing the dough into the corners of the pan.

Sprinkle the dough generously with salt, and add additional toppings as desired.

Bake 22–26 minutes, until golden brown. The internal temperature should be 200 degrees. Cool completely on a wire rack before cutting.

Serve aside a big lasagna or slice and top as a sandwich! Happy Baking!