Karen, you are probably right with the lamb but I also make it with beef.
I put the beef (lean, ground beef) in the pan with onions, peas, a chopped tomato, a fine chopped chili, garlic, olive oil, flat-leaf parsley, put the mashed potatoes (made myself from potatoes, milk and butter) on top and sprinkle the cheese on top (I like Holland Gouda for it).
Yummy

If I have any veggies left- carrots, celeriac, bellpeppers they can easily end in there.