Add sourdough starter to warm water first.
Stir them together until well combined.
Add flour, olive oil, and salt next.
Mix everything until no dry flour remains.
The dough will be sticky and wet.
Don't worry, that's exactly right.
Cover your bowl with a damp towel.
Let it sit at room temperature overnight.
The dough should double in size nicely.
This takes about 8 to 12 hours.
You'll see lots of bubbles forming.
Pour olive oil into your baking pan.
Transfer the dough gently into the pan.
Drizzle more olive oil on top generously.
Let it rest for 2 hours now.
The dough will puff up again.
Press your fingers into the dough deeply.
Make dimples all over the surface evenly.
Add fresh rosemary on top if using.
Sprinkle flaky sea salt generously everywhere.
Preheat your oven to 425°F (220°C).
Bake the focaccia for 25 to 30 minutes.
The top should turn golden brown beautifully.
The edges will get crispy and delicious.