For the tuna pizza, first preheat the oven to 200 °C. Cut the onions into thin slices.
Unroll half of the dough on one half of the greased baking tray. Unroll the rest next to it. Overlap seams a tiny bit and flatten to make a large pizza dough surface.
Spread tomatoes on top of dough. Sprinkle with salt, pepper and 1 teaspoon oregano. Spread onion slices evenly on top. Sprinkle with cheese and remaining oregano.
Bake tuna pizza in heated oven for about 20 minutes.