To make the dough: Combine the starter and water in a large mixing bowl.
Combine the flour with the starter, water, and remaining ingredients. Mix and knead — by hand or stand mixer — until the dough is smooth and elastic. If you're using a stand mixer, this should take 5 to 7 minutes on the lowest speed using the dough hook attachment. If you're kneading by hand, you'll need 12 to 15 minutes.
Place the dough in a bowl that's been lightly coated with olive oil, cover, and allow to rise for 60 minutes.
Gently fold the dough over three or four times, and let it rise for another 60 minutes.
Drizzle a generous 2 tablespoons olive oil into the center of a large sheet cake pan. Don't have a large sheet cake pan? See "tips," below.
Transfer the dough to the pan, and turn it over to coat it with the oil.
Gently stretch the dough into the edges and corners of the pan. As soon as the dough begins to shrink back, cover it, and let it rest for 10 to 15 minutes. Gently stretch the dough again, repeating the rest once more, if necessary, until the dough fills the pan.
Cover the pan and transfer it to the refrigerator to let the dough rise for 14 to 16 hours (overnight).
The next day, remove the pan of dough from the refrigerator and preheat the oven to 425°F for 30 minutes (if your kitchen is warm) to 60 minutes (in a cooler kitchen)
Just before you're ready to bake, gently dimple the dough at irregular intervals with your fingers, pressing down firmly but not abruptly; you don't want to deflate the focaccia too much.
Drizzle 2 tablespoons olive oil (or enough to collect a bit in the dimples), then decorate with your herbs & colorful veggies, or just sprinkle with rosemary and a bit of flaked sea salt.
Bake the focaccia for 20 to 25 minutes, until light golden brown.
Remove the focaccia from the oven. Allow it to cool enough for you to handle it comfortably, 10 to 15 minutes, then turn it out of the pan onto a rack.