Preheat oven to 350. Prepare a 2-quart casserole dish with cooking spray; set aside. In a skillet, cook turkey, onions, and bell peppers until no pink remains in turkey and vegetables are tender. Stir in mixed vegetables, soup, water, salt, and black pepper. Pour into prepared pan. Spoon mashed potatoes in mounds around edge of casserole. Bake, uncovered, for 25 minutes or until heated through. Top with cheddar cheese and bake 5 minutes longer.