Had some leftovers from our wonderful lamb dinner the other night, so put together a quick shepherds pie. Here's my go-to method:
Hopefully, you have some leftover meat, potatoes, vegetables (or just use frozen peas and/or corn), and gravy.
Chop and fry an onion and some garlic (there was enough garlic in the gravy from our leftovers, so I left that out this time.) Add minced leftover meat and chopped veggies (I had some green beans and broccoli that I threw in). Heat this up and add gravy to make a thick stew and put it in a casserole dish. (If you don't have gravy, use a cup of beef or other broth, a tablespoon of tomato paste and 2 tbsp of flour and just throw that in instead. Season with salt and pepper and maybe some parsley.)
Mash the potatoes if they are not already in that form, with lots of whatever makes them creamy. I like to add some cream cheese and/or butter and/or cream plus some Lawry's seasoned salt. Top the meat mixture with potatoes, grate some old cheddar or whatever cheese you want to use up, and pop it in a medium oven, uncovered, for twenty minutes or until the cheese has melted and the stew is bubbling up the sides. (You can assemble this in advance and just heat it up before you're ready to eat.)
Total comfort food! Additional gravy to pour over the top is a bonus.