In a large mixing bowl, combine active starter and water.
Add flours and salt. Mix four to five minutes using your hands or a dough whisk until no dry bits of flour remain.
Cover the bowl with plastic wrap and let sit at room temperature for 45 minutes.
Perform the first “stretch and fold.” A stretch and fold is done by gently lifting one side of the dough up and folding it down over itself. Turn the bowl a quarter turn and repeat this step until you have turned the bowl all the way around, in a full circle.
Cover the bowl and let dough rest 30 minutes at room temperature. Perform a second stretch and fold.
Cover the bowl and let dough rest another 30 minutes at room temperature. Perform a third stretch and fold.
Cover again and let dough rest three to five hours, until almost double in size. It should appear puffy and jiggly, with bubbles on the surface.
When the dough has bubbles on the surface and is almost doubled in size, line a 9×13-inch pan with parchment paper and coat with one tablespoon of olive oil. Transfer the dough into the oiled pan and rub another tablespoon of olive oil on top. Cover with plastic and let rest on the counter for one hour.
Preheat oven to 425 degrees Fahrenheit. Rub hands lightly with olive oil and using all ten fingers, press dough gently to dimple and stretch until it fits the pan.
Sprinkle generously with sea salt and toppings of your choice. Bake 35–40 minutes or until golden brown.
Transfer to a cooling rack at least 20 minutes before slicing.
Roast tomatoes with garlic and olive oil for 15 minutes and use to top the focaccia.
Photos by Mirele Schapira