Have to say to me these recipes for shepherd's pie are complicated!!!
Bring a tiny amount of oil in a pot to heat and cook up some diced onion and minced garlic until going translucent. Add ground lamb and move around in the pot to start colouring the meat. Add some water - couple of table spoons (if necessary, drain some of the oil from the pot first - some ground lambs produce a lot of fat) and beware of spitting oil when you do. Put the lid on for a little bit to help brown the meat. Add your choice of vegetables (no potato and I would steer away from spinach or green beans), mine is carrots, celery, capsicum (for colour) and mushrooms, all done to a small dice except the mushrooms which are just sliced. Add to the pot along with a good splash of Worcestershire sauce, a good blob or two of tomato paste, some dried herbs (I like parsley and chives here but you could also use thyme, rosemary (but go lightly), or sage quite well, maybe even some marjoram) and salt and pepper. I cook to taste and quantities are to looks, meat should dominate the ingredients. Transfer to a casserole dish when cooked.
Boil up enough potatoes to make a covering for your dish of choice. You want the mash to be about one third of the dish. Add some milk and butter to the mash while still hot and any seasoning required. Put mash over dish and bake casserole dish in oven until the potato is crisping up into little waves with brown peaks. Serve with salad and a good tomato chutney or sauce.
Yeah you can glaze the mash before putting it in the oven, but not really necessary. Cottage pie is very similar but as BT said, substitute the beef for the lamb and amend seasonings and vegetables to compliment.
These dishes are the sort that you can clear out your vege drawer on and really are just savoury mince topped with mash and then baked. Don't think of it as a dish but a couple of dishes put together. I'll use my bolognaise mince meat at times for a cottage pie. Sure you can get fancy with them, but this is rustic food designed to keep the farmer full and warm and be satiated. Meat and three veg.