Heat the tomato paste and strained tomatoes in a saucepan, simmer for 5 minutes at low heat. Meanwhile bring 150 ml water to the boil, add vegetables, cover and simmer over high heat for about 3 minutes, drain. Peel and halve the onion and slice into fine rings. Season sauce with salt, pepper and sugar. Cut flat bread in half horizontally once.
Spread with half of the sauce on each. Spread vegetables, onion and cheese on top. Place on 2 baking trays covered with baking paper. Bake the trays one after the other in a preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: level 4) for about 10 minutes. Serve garnished with marjoram