Clean the mushrooms and cut them in half. Peel and finely chop the onions. Chop olives. Cut pepperoni into fine rolls, except for a little bit for garnishing. Mix olives and pepperoni. Knead minced meat, breadcrumbs, mustard, half of the onion cubes and season with salt and pepper.
Form into 12 meatballs and fill with olive and pepperoni mixture.
Prepare rice in boiling salted water according to package instructions. Heat the oil in a pan and fry the meatballs for about 8 minutes while turning. Remove and keep warm. Fry the mushrooms in the frying fat for about 5 minutes.
After 2 minutes add the remaining onion cubes. Season with salt and pepper.
Add cooking cream and milk, bring to the boil and simmer for about 2 minutes. Wash the parsley, shake dry, pluck the leaves from the stalks and chop them, except for a little to garnish. Arrange meatballs, mushrooms and rice on plates.
Sprinkle with parsley and garnish.