Preheat oven to 400° F (205°C).
Melt butter in medium skillet over medium heat; add onion. Cook until lightly browned. Add cream cheese; stir until melted. Remove skillet from heat. Add stuffing; stir to coat with cream cheese.
Combine beef stroganoff and mushrooms in a large oven-safe skillet. Cover; cook over high for heat 3 minutes. Uncover; sprinkle with stuffing.
Bake for 15 to 18 minutes or until stuffing is lightly browned.Servings: 4