In a large bowl, combine the sourdough starter, bread flour, and 400 grams of water. Mix until the ingredients are just combined. The dough will be very wet and sticky.
Let the dough rest for 20-30 minutes. This resting period, known as autolyse, helps to hydrate the flour and develop gluten.
After 30 minutes, add the salt, remaining 10 grams of water, and olive oil. Knead the dough for 2-3 minutes. The dough will still be very wet.
Cover the bowl and let the dough rest at room temperature for 2-3 hours. During this time, perform a stretch and fold every 30-40 minutes. To do this, wet your hands, grab one side of the dough, stretch it upwards, and fold it over itself. Turn the bowl and repeat until you’ve stretched and folded all four sides.
After the bulk rise, generously oil a 9x13-inch sheet pan. Transfer the dough to the pan and gently stretch it out to cover the bottom. If the dough resists, let it rest for 20-30 minutes before stretching it again. Cover the dough with plastic wrap and refrigerate it overnight for 10-18 hours or leave it at room temperature for 2-4 hours if you prefer to bake the same day.
Remove the dough from the fridge 2 hours before baking to allow it to come to room temperature.
Preheat your oven to 450°F (230°C). Oil your hands and gently press the dough to create dimples all over the surface. Drizzle olive oil, sprinkle with your favorite herbs, and add a pinch of salt.
Bake the focaccia for 30 minutes or until it is deep golden brown.