Shepherd's pie has working-class roots and long served as a way for rural residents to stretch limited meat supplies. These "pies," made with minced meat, date back to at least the 1700s. Shepherd's pie includes meat, vegetables, onions and a thick gravy. Instead of crust, the dish is topped with mashed potatoes and, sometimes, grated cheese. Some cooks include any other leftovers that don't clash with the gravy, or they stretch the meat and veg even further by putting potatoes on the bottom of the dish as well as on top.
The main ingredient of shepherd's pie is minced meat (though meatless recipes also exist). Traditionally, cooks, especially in the U.K., minced lamb or mutton for their pies. When they used beef, which is usually a cheaper option for cooks in the U.S., they called the dish "cottage pie." Regardless of the name and the ingredients, the thick layer of potato on the top effectively holds the heat, bringing an extra level of warmth to the pie eaters.