Mix meatballs, ricotta cheese, parsley and tomato paste in a bowl until well blended. Filling can be made one day in advance and kept (covered) in the refrigerator.
Preheat oven to 400°. Pop the stems out of the mushrooms and arrange cavity side up on a greased rimmed baking sheet. Stuff mushrooms with meatball mixture, sprinkle with salt, pepper and Parmesan cheese, and bake for 12 minutes or until mushrooms release water and cheese is melted. Serve garnished with parsley leaves.
Serves 8 (as an appetizer)