Probably originating in Scotland and Northern England, Shepherd's Pie is a ground lamb, vegetable and mashed potato one-dish meal. Making it with ground beef turns the dish into Cottage Pie, which already doesn't sound as good as the lamb version! The dish appears to have originated as a way to use up left-over meat and vegetables, so by all means, use what you have on hand. You can use a lot of short-cuts, too - frozen mixed vegetables, instant mashed potatoes and grated cheese in a package. If you are starting from scratch, use what sounds most delicious to you: Fresh, more exotic vegetables cut and sauteed (leeks, carrots and green beans are one combination), freshly cooked mashed red potatoes with skin left on, topped with a mixture of freshly grated Swiss and cheddar cheese. It's all up to your time, tastes and energy, and any way you create it, it's a comforting, hearty dish after a long day!
Wash, peel (if desired) and place potatoes water, boil until fork tender. Mash with your favorite: butter, chicken broth, milk; salt and pepper to taste (or make those instant potatoes). Set aside.
Brown the lamb and chopped onion in oil or butter, remove from pan and set aside. Heat frozen vegetables in a bit of water (or sauté those fresh vegetables) in the same pan.
Add browned lamb back into the pan, add tomato sauce, Worcestershire sauce and salt and pepper to taste. Stir together. Add a bit of chicken or vegetable broth if filling is too dry.
Spoon the filling into an oven-proof pan sized so filling is about 3/4 to 1 inch deep. (Or, if you prefer, place filling in individual ramekins)
Spoon mashed potatoes on top of filling, taking care not to mix potatoes with filling. Brush a bit of melted butter on top of potatoes if desired.
Bake in 350-degree oven for about 25 minutes, until filling is bubbling and potatoes beginning to brown. It may be necessary to briefly broil for potatoes to brown. Sprinkle shredded cheese on top, return to oven for a few minutes until cheese is melted.