The night before you want to make the focaccia, you want to add water, oil, salt, sourdough starter, and 3 cups of flour to the bowl of a stand mixer and mix until combined. Note: This can be kneaded by hand.
Slowly add the remaining flour 1/4 cup at a time until a soft dough forms. Mine took 4 cups of hard white wheat flour.
Knead for 6-8 minutes.
Place the dough into a lightly greased bowl and cover. Allow to ferment on the counter overnight or up to 24 hours.
The next morning, prepare a 9x13 baking pan with olive oil spread in the bottom.
Gently turn the dough into the baking dish. Note: Take care not to deflate the dough.
Drizzle the top of the dough with olive oil.
Using the tips of your fingers, gently press into the dough making indentions as you go. Continue doing this until the dough spreads out into the pan completely.
You can add fresh or dried herbs, tomatoes, or coarse salt at this time.
Cover and allow to rise for 2-3 hours.
Preheat oven to 400 degrees F.
Bake the focaccia for 20-22 minutes.
Slice and have as a side with dinner or you can top with pizza topping and put under the broiler for one of the most delicious pizzas ever!