Peel and finely dice 1 onion. Chop the tomatoes finely. Knead minced meat, breadcrumbs, onion, tomatoes (up to 1 tablespoon) and egg. Season with salt, pepper, paprika and oregano.
Form 12 meatballs from the mince mixture. 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.
In the meantime, boil 750 ml of water with 1 teaspoon of salt and remove from the heat. Add milk and stir in the puree powder. Let stand for 1 minute, stir again. Peel 1 onion, cut into thin rings.
Heat the oil in a pan. Fry the onion rings until golden brown. Remove the meatballs, place them on a plate and garnish with oregano. Sprinkle the rest of the tomato pieces on top. Pour the puree into a bowl.
Add the onion rings. Serve mashed potatoes and meatballs together. Serve with a fresh salad.