Roast beef and mashed potatoes are a generational favorite extending from my childhood to my grandchildren. This recipe originated at the Iowa State Fair. It can be used with leftovers, or you can buy prepared roast beef with gravy and use instant mashed potatoes. If you use leftover roast beef, remember to slice it against the grain to make it easier to eat. The servings can be scooped liked ice cream in a bowl or layered in a parfait cup.