Soak rolls in cold water. Peel and finely chop the onion. Squeeze the bread well. Knead minced meat, roll, onion, egg and aiwar. Season with salt and pepper. Form 12 meatballs from the mixture with moistened hands.
Line a baking tray with baking paper. Place the meatballs on top and bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 20 minutes. Drain mozzarella and cut into 12 slices. 5 minutes before the end of the cooking time, cover each meatball with 1 slice of mozzarella. Wash the tomatoes, dab dry and cut in half. Take the meatballs out of the oven and cover each with 1/2 tomato. Served with potato salad