You can create a delicious shepherd's pie using your leftover mashed potatoes as the topping. Start by sautéing the ground turkey with onions and seasonings, then mix in the green beans. Place the turkey mixture in a baking dish, top with the mashed potatoes, and bake at 375°F for about 30 minutes. This dish is hearty and can serve as a comforting meal with around 450 calories per serving.