Stir the yeast and sugar into the warm water and leave for 5 minutes until foamy. If it does not foam, the yeast is dead and the dough will not rise, so start again with a fresh sachet.
Mix the flour and salt in a large bowl, add the yeasty water and the olive oil, and bring together into a rough dough. Knead on a work surface for 5-8 minutes until smooth and springy.
Shape into a ball, put it back in the bowl with a wipe of oil round it, cover with a damp tea towel and leave somewhere warm for about an hour until doubled in size.
Meanwhile make the sauce: put the tomatoes, garlic, oregano and a pinch of salt in a small pan and simmer for 10 minutes until thick and no longer watery. Season to taste, it should taste slightly too strong on its own because the dough will dilute it.
Heat the oven to its highest setting, 240C/220C fan or hotter, with a baking sheet inside to heat up. Push the risen dough out on a floured surface into a 30cm round, pressing from the middle outwards and leaving a slightly thicker rim.
Slide the base onto the hot sheet, spread over the sauce, scatter the mozzarella and drizzle with a little oil. Bake for 10-14 minutes until the edges are puffed and golden and the middle is bubbling. Rest for a couple of minutes before cutting.