What I lovingly think of as Shepherd's Pie has only some basics in common with the original OR the common one from the '60s...
Unlike the original, it's not a mutton stew covered with mashed potatoes...
And unlike the '60s version, it's not a dumping ground for all of the contents of various Tupperware containers from the past week's meals...
So, if you're willing to start it from scratch, I think you'll be awfully happy.
I make a basic béchamel: equal parts butter and flour cooked for about 3 minutes on low, to which I add milk... you choose just how thick you want the sauce... but 3 T each butter and flour to 1½ C of milk is a good place to start.
Let that simmer on low while you prep the meat. Cook some ground meat (beef or pork or turkey or dark meat chicken or all of the above) until it JUST starts to lose its pinkness. Add a little salt and TONS of black pepper. To this, I like to add some sautéed onions, and lots of frozen peas, corn, and maybe some sautéed carrots and celery.
Combine with some of the thickened béchamel. Check for seasonings (and yes, add whatever else you like... this is just a basic framework) and consistency... not too much béchamel, but not dry.
Spread this mixture in a pyrex-type baking dish (you want the filling between 1 and 2 inches thick).
Top with mashed potatoes (NOT too wet... make these a little on the dry side, i.e., less dairy).
Heat in a 350° oven until hot enough for you. The timing will all depend on how hot your filling and potatoes were to begin with, but start with 20 minutes. If you want the top to brown attractively, you can brush with a bit of egg, or top with some Parmesan and broil for a minute or 2...
But I kind of like the soft UN-crispyness of the dish. You don't get much more "comfort food-y" than this.
