Before making the dough, be sure to feed your sourdough starter at least twice, allowing it to reach peak activity level.
In a large mixing bowl whisk the called-for active starter, lukewarm filtered water, olive oil, and salt together. Start on the lower end of the water range, and add more after adding flour if needed to achieve desired dough texture
Add the called-for flour to the mixture and stir until thoroughly combined. Note that focaccia dough is slightly more wet than typical sourdough bread dough.
Cover the bowl with a cloth and allow to sit (bulk ferment) at room temperature (70-75 degrees is optimal) for about 4 hours, or until it has risen and doubled in size.
During the first two hours of bulk ferment, perform 3 to 4 sets of “stretch and fold” every 30 minutes. Then allow the dough to sit undisturbed for the final 2 hours.
Once the dough doubles from its original size, transfer into a well-oiled baking pan/sheet.
Using wet or oiled hands, gently push and pull the dough into the corners of the pan (or to spread out some on your shallow baking sheet), but it will spring back and resist. Encourage it, but don't force it. It will continue to spread on its own with time.
If baking the same day, allow the dough to rise at room temperature (about 2 more hours) until it doubles in size again before next step. OR proof in the refrigerator overnight. If proofing overnight, allow the dough to warm up at room temperature for a couple of hours the following day and doubles in size before the next step.
Preheat the oven to 450 degrees Fahrenheit.
Using wet or oiled hands, press your fingers into the dough repeatedly in many places (all the way to the bottom of the pan) until the dough is dimpled and bubbly.
Drizzle olive oil over the top of the dough, sprinkle with coarse salt, and add toppings if desired. (You can also drizzle oil over the top first, and then dimple it with your fingers)
Bake at 450 degrees F for about 25 minutes, or until golden brown.
Once the bread has finished baking, allow the bread to cool for a few minutes inside the pan before transferring it onto a cooling rack.
Serve and enjoy warm, or use it within 2 days for optimal freshness.
Store in an air-tight container or covered in plastic wrap, and re-heat on 375F for 10 minutes if desired.