Add sourdough starter, water, and salt to a bowl. Mix with a spatula or fork until well blended.
Add flour and mix until a dough forms.
Cover with plastic wrap or a damp towel and let rest 30 minutes.
Do a "pinch and fold"—pinch the outer edge of the dough ball and fold it to the centre. Do this 8-10 times around the dough ball.
Gently rub the top of the dough ball with a splash of oil.
Cover and let rest 4-18 hours, until doubled in size.
Once doubled in size, drizzle about 2 Tbsp oil into a 9x13 baking dish. Optionally, you can spray or grease the sides of the dish to help prevent sticking.
Scoop dough into baking dish. It should form an oval. Fold the dough so it roughly looks like a rectangle and flip the dough so the seam side is down.
Cover and let rest 4-6 hours. During this time you could prep any optional toppings that need prepping, such as roasting garlic.
Heat oven to 425℉.
Rub oil on your hands and press down on the dough, creating dimples across the top. Sprinkle with flaky salt and add any toppings you wish.
Bake for 25 minutes until golden brown.
Let rest at least 20 minutes before slicing. If it's hot while slicing, moisture may evaporate, leaving a dryer, denser bread.