1. Heat oven to 375 degrees. In medium saucepan, combine turkey, green beans, soup, and milk; mix well. Cook over medium heat until mixture is hot, stirring occasionally. Remove from heat. Add cheese; stir until melted. Pour into ungreased 2-quart casserole. Top with prepared mashed potatoes.
2. Bake at 375 degrees for 10 minutes. Sprinkle with onions; bake an additional 3 to 5 minutes or until mixture is bubbly and onions are warm.