In the "old days" shepherd's pie was made with roast beef, gravy, vegetables, and mashed potatoes--the leftovers from Sunday dinner. I used to make it when I had leftover mashed potatoes, but then later I went to all the trouble of making the mashed potatoes so that we could have Shepherd's Pie. The key to the flavor is in browning the beef and carmelizing the onions to develop richness and then making a gravy with the results. Heavenly!