In a large bowl or stand mixer bowl combine the starter, quick yeast, water and flour. Mix on low speed for a few minutes to incorporate, scrape down the sides and increase the speed for 8 minutes.
Once the dough is wet and sticky, not soupy but more of a batter consistency, add in more flour part at a time if it seems too wet.
Add in the salt and continue mixing for another 3-5 minutes.
Once the dough begins to ball together and pull away from the sides of the bowl it is ready to rise.
Grease the sides of the bowl to prevent the dough ball from sticking, cover the bowl with a damp towel, bees wax wrap or plastic wrap and let rise in a warm place until doubled. About 1-2 hours.
Position one oven rack at the bottom of the oven and one in the centre of the oven. Preheat the oven to 500 °F
Lightly oil all sides of the cast iron skillet with about 2 tbsps of oil and scrape the dough into the skillet. With your fingers press the dough out from the centre gently popping some of the air bubbles.
Top with the remaining oil and dimple the dough pressing the oil into the dimples.
Let the dough rise until doubled for about 1 hour. Dimple the surface one more time, use a spoon to redistribute any large pools of oil. Sprinkle the top with the flaky salt and any other toppings that you may want to add.
To a 9x9 or 9x13 pan add at least an inch of cold water, this will go on the bottom rack of the oven to create steam during the bake.
Set the pan of water on the bottom rack and get the skillet on the centre rack. Bake for 15 minutes, do no open the door during this time.
Once the 15 minutes is up, rotate the focaccia, reduce the heat to 450 °F nd bake for another 5-15 minutes until the top is completely golden brown.
Remove from oven and let rest for 10 minutes. Remove from the skillet and let cool completely (almost completely) before devouring.