Grease a baking tray for the tuna pizza.
Divide the yeast dough into two portions, form each into a narrow round pizza, raise a thick edge and place on the tray.
Peel the onions, halve them and cut them into very fine half rings. Heat the olive oil in a frying pan and sauté the onions in it at a low temperature.
Peel and squeeze the garlic and add it. Season with salt, pepper and oregano. Remove the tuna from the can, drain and roughly chop with a fork.
Rinse the parsley, pluck from the stems and chop finely. Mix with the tuna and the onions. Spread the tomato cubes evenly on the yeast dough, season with salt and pepper.
Pour the tuna and onion mixture on top and bake the tuna pizza in the preheated oven at 200 degrees on the lowest rack for 25 minutes.