Mix the sourdough starter, flour, and 400 g of water until combined. At this point, it will feel like a wet dough.
Let the dough sit for 20-30 minutes. After 30 minutes, add the salt, remaining water, and oil. Knead the dough for 2-3 minutes (the dough will be very wet).
Bulk rise: Let the dough rest at room temperature for 2-3 hours, performing a stretch and fold every 30-40 minutes.
After the bulk rise, oil a 9x13 sheet pan and transfer the dough to it. Gently stretch the dough, but let it rest for 20-30 minutes if it resists.
After an hour, oil the top of the dough and cover with plastic wrap. Refrigerate overnight (10-18 hours).
Remove from the fridge 2 hours before baking and let it come to room temperature.
Preheat the oven to 450°F. Oil your hands and gently press down on the focaccia dough. Drizzle olive oil, herbs, and salt on top.
Bake for 30 minutes or until deep brown in color.