Peel and finely chop the onion. Knead with mixed mince, 1 egg, breadcrumbs and mustard. Season with salt and pepper. Form 4 large meatballs from the minced meat mixture.
Fry the meatballs in 2 tbsp. oil for about 10 minutes, keep warm.
Clean the mushrooms, possibly wash and quarter them. Wash and slice the tomatoes. Heat 1 tablespoon of oil in a frying pan. Fry the tomatoes briefly in it and take them out. Fry the mushrooms, season with salt and pepper. Deglaze with cream, bring to the boil. Dice cheese, melt in it, simmer for about 5 minutes.
Wash the parsley, pluck. Arrange the meatballs with mushroom sauce, tomatoes and parsley.