Preheat oven to 350. In a 2-quart casserole dish , firmly press hash brown potatoes evenly across bottom to form a shell; set aside. Meanwhile, in a skillet, heat oil over medium heat. Add turkey, onions, and bell peppers. Cook until turkey is longer pink and vegetables are tender. Stir in water, gravy mix, and salt; bring to a boil. Add mixed vegetables; reduce and cook, uncovered, 5 minutes. Remove from heat. Stir in 1/2 cup cheddar cheese. Spoon in hash brown potato shell. Bake, uncovered, for 15 minutes or until heated through. Top with remaining cheddar cheese. Bake, uncovered again, for 5 minutes more.