Drain tuna and pineapple separately on a sieve. Cut the pineapple into pieces. Cut ham into wide strips. Peel and slice the onion. Cut flat bread in half and place it on a baking tray lined with baking paper with the cut surface facing upwards. Spread the tomato pieces on the bases and sprinkle with marjoram, pizza spice, salt and pepper.
Cover one half of the bread with tuna and onion rings, the other half with ham and pineapple. Sprinkle tuna and onion pizza with 3/4 of the cheese. Sprinkle the remaining cheese on the ham and pineapple pizza. Bake both flat bread halves in the preheated oven (electric oven: 200°C/ gas: level 3) for about 15 minutes. Serve the pizza halves garnished with thyme and basil as desired