Tip the flour into a very large bowl, forming a heap with a “well” in the top; put the sourdough starter in the centre with a little water and mix until it dissolves. Add the salt, olive oil and sugar; then a little at a time, using a fork, incorporate the flour from around the edge into the liquid, gradually adding the rest of the water. Knead with your hands, using a few handfuls of flour, until you obtain a smooth, soft ball of dough. Cover with a cotton cloth and leave to rise for about 2 hours. After this time, knead the dough (which in the meanwhile will have doubled in size) again and then leave it to rise for another hour. Transfer it onto the work surface, sprinkled with a good handful of flour, and use a rolling pin to roll it out (bearing in mind the size of the baking tray on which you will cook the pizza). Transfer the pizza base onto the baking tray, greased with olive oil, and spread it out with your hands.
Put the peeled tomatoes through a food mill into a bowl and season with olive oil, salt, pepper and oregano. Distribute the sauce over the pizza dough, then sprinkle with the grated pecorino cheese and bake in a preheated oven, at 220°C, for 20 minutes.