In days past, when Sunday dinner was routinely a roast with gravy, a meal of leftovers inevitably followed. One appealing roast reincarnation was as shepherd's pie (or "cottage pie") ― cold meat and gravy with a top crust of mashed potatoes, baked until browned and bubbly. One reference I came across recently claims that the dish first appeared in the 1870s with the advent of a mincing machine to grind the meat. In The Boston Cooking-School Cook Book, by Fannie Merritt Farmer (originally published in 1896 and still surviving as The Fannie Farmer Cookbook), the recipe for shepherd's pie is brief to the point of barren and uses roast beef. In other old cookbooks, the meat is often lamb.
Without leftovers from a roast, though, how do you create this comfy dish? My choice is to make a richly browned, deeply flavored onion gravy, poach moist meatballs in it, then nestle the mixture in a wreath of mashed potatoes. I think Fannie would approve.