Preheat oven to 425 degrees F. Brown beef in a deep skillet. Halfway through browning, add chopped onion; drain fat. Add paprika, salt, pepper, gravy mix and water to skillet. Mix well over medium heat. Add vegetables. Cover and simmer while potatoes boil.
Once the meat mixture is simmering and the veggies are soft, spread mixture in bottom of casserole dish (8x8). Spread mashed potatoes on top of meat, covering to the sides of the dish. If desired, top with pats of butter and shredded cheese (optional).
Bake uncovered until meat mixture bubbles on the edges and potatoes are brown on top.