This easy casserole is made with leftover roast beef and mashed potatoes. If you don't have leftover mashed potatoes, make your favorite mashed potatoes using about 1 1/2 to 2 pounds of potatoes. Cottage pie or shepherd's pie, what's the difference? A cottage pie is often called a shepherd's pie, but there's a difference. A true shepherd's pie is made with lamb, while a cottage pie is made with beef.