Peel the onions and clean the mushrooms. Cut both into cubes. Fry them together in a good dash of hot olive oil. As soon as the onions are translucent, add the meatballs and tomato paste. As soon as the onions and mushrooms have turned a nice color, pour the vegetable stock over them. Add the ketchup and season with salt, pepper and paprika powder. Let the sauce simmer a little longer.