Clean the mushrooms and, depending on size, halve or slice them. Peel onion and garlic. Cut onion into thin slices, press garlic through a garlic press. Clean, wash and drain the tomatoes and spring onions. Cut the white end of the spring onions into approx. 2 cm long pieces, the rest into fine rings.
Cut the olives into slices. Drain the tuna. Heat the fat in a pan. Sauté the mushrooms, onions and spring onions briefly while turning. Add garlic and season with salt and pepper. Unroll the pizza dough with the baking paper and place on a baking tray. Sprinkle with half of the cheese and cover with tuna, tomatoes and mushroom-onion mixture. Sprinkle with olives, oregano, salt, pepper and remaining cheese. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for about 20 minutes.
Add garlic and season with salt and pepper. Unroll the pizza dough with the baking paper and place on a baking tray. Sprinkle with half of the cheese and cover with tuna, tomatoes and mushroom-onion mixture. Sprinkle with olives, oregano, salt, pepper and remaining cheese. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for about 20 minutes. Serve immediately garnished with basil as desired