Stir together the flour, yeast and 1 tsp salt in a bowl. Measure 150ml of warm water in a jug and add 2 tbsp of olive oil. Gradually mix the water into the flour until a dough forms. Tip out onto a lightly floured worksurface and knead for 8 minutes, until smoother and more elastic. Put in an oiled bowl, cover with a tea towel and leave to rise for 1 hour.
Drain the tomatoes in a sieve for 10 minutes (save the juice for a bloody mary), then whizz the pulp in a food processor or blender with the garlic and 1 tbsp of olive oil. Stir in the oregano, ½ tsp of sea salt and lots of black pepper.
Generously brush a baking tray (approximately 25cm x 35cm) with olive oil. Punch the dough down in the bowl a few times to knock out the air, then tip into the baking tray. Use your hands to gently push and stretch the dough to fill the tray. Leave in a warm place for 30 minutes until the dough has started to puff up.
Heat the oven to 220C/fan 200C/gas 7. Spoon 3-4 generous tbsp of sauce over the dough and spead out in a thin layer (freeze the rest). Scatter over the mozzarella, onion, peppers and pepperoni. Drizzle with another 1 tbsp of olive oil and season with lots of cracked black pepper. Cook for 25 minutes or until the dough is golden underneath and the toppings are bubbling. Slice to serve.