Combine 1 cup of active sourdough starter and 1 1/2 cups of warm water in your large mixing bowl. Stir until the starter dissolves. Add 4 cups of all-purpose flour and 1 1/2 teaspoons of fine sea salt. Mix everything together using a dough scraper or your hands until a shaggy, sticky dough forms. Pour in 1/4 cup of extra-virgin olive oil and incorporate it evenly.
Cover the bowl with a kitchen towel or plastic wrap. Let the dough rest at room temperature for about 3 to 4 hours, or until it has noticeably risen and looks airy.
Every 30 minutes during the first 2 hours of bulk fermentation, perform a gentle stretch and fold to strengthen the dough. Wet your hand to prevent sticking, then reach under the dough, stretch it upward, and fold it over the rest. Turn the bowl and repeat 3-4 times.
Generously grease your 9x13 inch baking pan with olive oil to prevent sticking and help create a crispy, golden crust.
Transfer the dough to your prepared pan. Using oiled fingers, gently press and stretch the dough to fill the pan evenly. Be gentle to keep the air bubbles intact.
Cover the dough again and let it rise for another 1 to 2 hours, or until it puffs up and fills the pan.
Use your fingertips to create characteristic dimples all over the surface of the dough. These pockets will hold olive oil and toppings, infusing the bread with flavor as it bakes.
Drizzle additional olive oil generously over the dough. Sprinkle 1 tablespoon of fresh rosemary leaves (if using) and flaky sea salt evenly over the top.
Preheat your oven to 450°F (230°C). Bake the focaccia on the middle rack for 20 to 25 minutes, or until golden brown and crisp on the edges.
Remove the focaccia from the oven and transfer it to a wire rack to cool slightly. Enjoy warm drizzled with more olive oil, or cool completely before slicing.
