Bring tomatoes to the boil in a pot and simmer for about 3 minutes. Season with salt, pepper, paprika and 1 pinch of sugar. Drain the tuna. Peel onions and cut them into rings.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Roll out dough and halve crosswise if necessary. Press into two pizza trays (each approx. 24 cm Ø) or place on a baking tray.
Spread tomato sauce on the pizzas. Cover with tuna and onions. Sprinkle with cheese. Bake in a hot oven on the lowest shelf for 15-20 minutes.