Mix flour, water, sourdough starter, salt and olive oil into a dough and knead until smooth. Cover and let it ferment slowly for about 24 hours.
Preheat the oven well with a baking tray, stone or baking steel inside.
Divide the dough into two portions and stretch each one into a medium pizza. Spread tomato sauce over the surface, then add mozzarella, mushrooms, capers, olives and onion.
Bake for about 10–12 minutes until the crust is golden and crisp and the cheese has melted.
Remove from the oven, drizzle lightly with olive oil if desired, and finish with a handful of fresh rocket. Let the pizza rest briefly before slicing and serving.