Add water to a large mixing bowl followed by the sourdough starter and olive oil then stir with a fork or dough whisk to combine everything and break up the starter.
Add the flour and salt to the bowl and mix with the fork/dough whisk at first then switch to mixing by hand until a wet sticky dough forms after about 5 minutes. When mixing by hand scoop from the bottom and pull the dough up towards the center to make sure there are no pockets of dry flour.
Scrape down the sides of the bowl with a dough scraper and then cover the bowl with a shower cap, plastic wrap, or beeswax wrap. Don't use a kitchen towel for this as the dough can dry out and form a crust on top.
Let the dough bulk rise at room temperature until it doubles in volume and is very airy and jiggles when you gently shake the bowl. The bulk rise timing will depend on how warm and humid the surrounding atmosphere is. At warmer/more humid areas, the dough can rise within 4-8 hours. At cooler temperatures, it may take upwards of 12-18 hours, in which case it can be timed to bulk rise overnight. The timing can also be slowed down by refrigerating the dough after the halfway point of the bulk rise.
After the bulk rise, the dough can be placed into a plastic zip bag and stored in the fridge for up to 3 days before proceeding. The longer the dough stays in the refrigerator the more pronounced sour flavor it will have and after 3 days it will start to lose its strength and air bubbles.
Line a quarter sheet pan with parchment paper so that the paper comes up the sides of the pan.
Pour the half of the olive oil onto the parchment paper then drop the focaccia dough onto the oil. Flip the dough over a couple of times to coat it entirely in the oil. Then gently stretch it out towards the corners of the pan.
Cover the pan with plastic wrap or place it in a proofing bag and let it proof in a warm spot until it is puffy with air bubbles, about 1 hour.
About 30 minutes before the focaccia is done proofing preheat the oven to 450˚F.
Oil your hands and press your fingertips straight down into the dough to create dimples on the surface.
Add toppings to the focaccia at this point or simply sprinkle the top with flaky sea salt.
OPTIONAL STEP: Cut two pieces of aluminum foil to about 18” in length. Lay them on top of each other and fold them together from one of the long edges a few times to create one large piece of foil. Cover the sheet pan with the foil leaving enough space for the focaccia to puff up and tightly wrap the edges to hold in the steam.
Bake for 15 minutes covered with the foil then carefully remove the foil. The focaccia will puff up but still look uncooked at this point. Bake it for 30 more minutes uncovered until the top of the focaccia is nicely browned.
Immediately transfer the baked focaccia to a cooling rack to prevent the bottom from becoming soggy.