Clean the mushrooms, wash and halve them. Peel onions and garlic and chop finely. Heat 1 tablespoon of oil in a saucepan, fry the mushrooms while turning them, add half of the onions and garlic, fry over low heat until translucent.
Add the rice and briefly sauté. Gradually add wine and stock, stirring from time to time. Add the next portion of liquid only after the rice has absorbed the liquid. Cook for a total of 30-35 minutes.
In the meantime, remove the tomatoes from the oil and cut into cubes. Knead minced meat, remaining onions and garlic, tomatoes, egg, breadcrumbs and mustard. Season with salt and pepper. Cut mozzarella into 8 cubes.
Shape the mince into 8 meatballs and fill each with a cube of cheese. Heat 2 tablespoons of oil in a frying pan and fry the meatballs for approx. 10 minutes, turning them over. Wash the oregano and shake dry. Pluck the leaves from the stalks, except for something to garnish.
Stir the parmesan into the risotto. Season to taste with salt and pepper. Arrange meatballs and risotto on plates. Sprinkle with pepper and oregano and garnish with oregano.