I like to make cottage pie and serve freshly made. It could be frozen but I would recommend just making and freezing the base. You could then defrost this at a later date to use with some leftover mashed potatoes. I don't freeze it with the potatoes already on top, I never managed to do a good job of freezing potatoes, but if you've managed to figure out a trick to it, by all means feel free (and let me in on the secret while you're at it).
Now so I'm told cottage pie contains minced beef, if it's shepherd's pie you're after then you need minced lamb. You won't find it here though, that stuff costs a fortune at the minute and really who would waste lovely lamb by mincing it.
For old recipes like this I usually consult one of two books, either Mrs Beeton's Everyday Cookery or Mrs Beeton's Book of Household Management. You can get some fantastic cheap and tasty recipes from these books although you may need to play with them a little, sometimes substituting ingredients as well. Food didn't globe trot back then as it invariably does today so everything was locally produced and in season. This always makes for the best tasting and cheapest food.
If you ever come across one of these books second hand or on ebay I would recommend picking it up, it's a great read and a real eye-opener, although I haven't yet had any need for her advice on hiring a full household staff.
1lb Minced Beef cooked
1 small Onion
1 tbls Oil
1/2 pint gravy
2lb mashed potatoes cooked
milk 2 tbls
Salt and Pepper
Saute onions in oil until soft and lightly browned, add to cooked minced beef in a large bowl, along with gravy and mix well. Season to taste.
Place in a pie dish and cover with mashed potatoes.
Brush the surface of the mash potatoes with a little milk and bake in the oven at 180C (350F) for 30-40 minutes, taste with a meat thermometer or a skewer inserted into the pie for the count of ten and then carefully touched to the lip, carefully.
Serve with a side of vegetable of your choice for a complete meal. This amount will feed six comfortably, you may find it will make enough for two meals.
To use up any leftovers I usually remove the mash (don't be too picky about it, just spoon off some of the potatoes and re-heat separately with a little butter and milk), reheat the minced beef and gravy in a pan, adding finely diced carrot and frozen peas and you have stewed bugs and onions and mashed potatoes.
The name alone pretty much guarantees that kids will eat almost exactly the same meal as last night, without any complaint whatsoever.
For more recipes using ground meat visit Jenn at Frugal Upstate and take some time to look through her Frugal Food Series.