Soak the stale bun in water. Finely chop the onions, garlic and basil. Cut the mozzarella into small cubes.
Season the minced meat with salt, pepper and mustard, squeeze out the soaked roll and add to the mince with the egg, half of the onions and half of the garlic. Mix everything well and shape into small balls, put a mozzarella cube in the middle of each ball.
Mix the tomatoes with the tomato paste, the olive oil (2 tablespoon), the red wine, a dash of white wine vinegar, salt, pepper, a pinch of sugar, the chopped basil, the remaining onions and the garlic. Heat 2 tablespoons of olive oil in a pan and briefly sear the meatballs. Place the meatballs in a baking dish, pour the cold-stirred tomato sauce over them and sprinkle with buffalo mozzarella cubes. In the oven for 25 minutes at 180 degrees (convection).
This goes well with pasta, rice or a baked polenta slice.