Heat oven to 375°F. In 2-quart saucepan, mix turkey, green beans, soup and 1/3 cup of the milk. Cook over medium heat 6 to 8 minutes, stirring occasionally, until mixture is hot.
Make mashed potatoes in microwave as directed on box using water, remaining 2/3 cup milk, butter, salt and mashed potato mix.
Remove turkey mixture from heat. Stir in cheese until melted. Pour into ungreased 2-quart casserole. Top with mashed potatoes.
Bake uncovered 10 minutes. Sprinkle with onions; bake 3 to 5 minutes longer or until mixture is bubbly and onions are warm.