Make a light, airy focaccia with crispy edges and tender crumb using sourdough starter. This recipe takes about 6-8 hours total (mostly hands-off rise time) and is perfect for beginners looking to use their starter in something other than bread. You'll end up with a beautiful, dimpled flatbread that's ideal for sandwiches, dipping, or enjoying on its own.
If your starter isn't already active and bubbly, feed it 4-6 hours before you plan to start baking. You'll need it at its peak (doubled in size, bubbly, and passing the float test).
In a large mixing bowl, combine the active sourdough starter, lukewarm water, and olive oil. Stir together, then add the bread flour, salt, and honey. Mix with a wooden spoon or your hands until no dry flour remains. The dough will be quite wet and shaggy—this is normal.
Let the dough rest for 30 minutes, then perform a stretch and fold: wet your hand, grab one side of the dough, stretch it up, and fold it over itself. Rotate the bowl 90 degrees and repeat 4 times total. This builds strength without kneading. Repeat this process 3 more times at 30-minute intervals (4 sets of stretch and folds total over 2 hours).
After your final stretch and fold, cover the bowl with a damp tea towel or plastic wrap and let it rise at room temperature for 3-4 hours, or until the dough has roughly doubled in size and looks puffy with visible bubbles on the surface. In a cooler kitchen, this may take longer.
Generously coat a 9x13 inch baking pan (or similar large rectangular pan) with olive oil—use about 3 tablespoons and make sure to get the corners and sides well-oiled. This creates the crispy bottom crust.
Gently turn the risen dough out into your prepared pan. Use oiled hands to carefully stretch and press the dough toward the corners, being gentle to preserve the air bubbles. If it resists, let it rest for 10-15 minutes and try again. Get it roughly to the edges but don't worry if it doesn't fill the pan completely yet.
Cover the pan with a damp towel and let the dough rest for 45-60 minutes. It should puff up and relax further into the pan. Meanwhile, preheat your oven to 425°F (220°C) during the last 20 minutes of this rise.
Once puffy, drizzle about 2 tablespoons of olive oil over the top of the dough. Using your fingertips, press straight down (not through to the bottom) all over the dough to create deep dimples. This is the signature focaccia look. Gently press the dough to the edges one final time if needed.
Sprinkle the fresh rosemary leaves evenly across the dimpled surface, pressing them gently into the dough so they don't burn. Finish with a generous sprinkle of flaky sea salt all over the top.
Place the pan in the preheated oven and bake for 25-30 minutes, until the top is deeply golden brown and the edges are crispy and pulling away from the sides. The internal temperature should reach about 200°F (93°C) if you want to check with a thermometer.
Remove from the oven and let cool in the pan for 5 minutes, then transfer to a wire cooling rack. For the best texture, let it cool for at least 15 minutes before slicing. Drizzle with a little extra olive oil if desired. Slice into squares and serve warm or at room temperature.