Mix the dough according to the directions on the pack. Knead it on a worktop, or in a standing electric mixer with a dough hook, for about 8-10 minutes. Coat the dough in the olive oil, place in a large bowl, cover with a clean tea towel and leave to rise in a warm place until doubled in size. It should take 1-2 hours. Preheat the oven to gas 2, 240°C, fan 220°C. Put a pizza stone or flat metal baking tray on the middle rack. Sprinkle a flat metal baking sheet generously with the plain flour - this will help the pizza glide from the sheet onto the hot metal tray in the oven. Divide the dough into four balls. Roll out one at a time, about 12in (30cm) across, and place on the floured sheet. Spread a quarter of the tomato sauce on the pizza, then top with one quarter of the cheese, one quarter of the pepperoni and some onions. Open the oven door and slide the pizza from the flour-coated sheet onto the hot stone or tray. Cook for 5 minutes or until golden and crisp around the sides. Repeat with the other three dough balls. Slice and serve immediately.