Too late for tonight due to the time difference (westernU.S> and all that), but this is really good for something different. It assumes you already have the meatballs made...
Add 1 tablespoon butter to the skillet, saute mushrooms and remove from heat. Stir in flour and ketchup; add broth gradually. Add onion soup mix, bring to a boil, then reduce heat and add meatballs. Stir in sour cream; heat through but do not boil. Serve with hot buttered noodles, mashed potatoes, or rice.