Peel and thinly slice the onions. Drain the tuna. Roll out one half of the dough on one half of a greased baking tray and cut it off. Unroll the rest beside the strip.
Allow the seam to overlap slightly and press on. Spread the tomatoes on the dough. Sprinkle with salt, pepper and 1 teaspoon of oregano. Spread onion slices, tuna fish and olives on top.
Sprinkle everything with cheese. Season again with 1 teaspoon of oregano. Bake the pizza in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 20 minutes. Take out and cut into pieces.