Preheat oven to 350°. Lightly grease or spray a 2 quart casserole dish ( I use a 9" x 11").
Combine the roast beef, gravy, and vegetables. Pour in the bottom of the casserole dish. Spread the mashed potatoes on top. Sprinkle cheese on top of the potatoes.
Bake for 25-30 minutes. If the casserole was chilled before cooking you may need to add another 5-10 minutes to the cooking time.