Combine the meat, gravy, and vegetables in a large saute pan and cook over medium heat just until everything is heated through. Season with salt and pepper.
Pour the heated mixture into a large glass pie plate or a round glass baking dish. Add the horseradish to the heated potatoes. Spread the mashed potatoes over the meat mixture, smoothing with a spatula and spreading to the edges of the pie dish.
Sprinkle the top with the bread crumbs and the Parmesan. Dot the top with butter. Bake for 45 minutes on the top shelf of the oven. Raise the oven temperature to broil and broil just until the crumbs are browned, about 1 minute. Serve.