My cooking guru, Alton Brown, says that Shepherd's Pie is made with shepherd, or failing that, then mutton or lamb. If you use anything else it should be called a Cottage Pie.
Here's a snippet of the recipe from the Good Eats 3: The Later Years:
I, sadly, had no shepherd in the freezer nor any lamb or mutton. I went with plain ole ground beef so I made a Cottage Pie.
The other ingredient I was missing was carrots which is odd because I almost ALWAYS have carrots in the fridge. (I blame CPA Boy who kept taking them to lunch.)
But I did have some zucchini and winter squash from a prior dinner (stuffed peppers in the crock pot). I sauteed them first because I thought they might release a little too much liquid though it ended up that they didn't.
As far as I can tell zucchini and winter squash taste basically the same. They are no real replacement for carrots but it is a good idea to get a few more veggies into a meal.
I put the sauteed squashes aside and started on the onion. I add onion to almost everything (whether the recipe calls for it or not) so naturally I think I should have used more.
Then comes time to add the shepherd substitute! Ground beef from Costco (which, sadly, does not seem to carry shepherd OR mutton). They may carry ground lamb. I'll need to check next time.
Then after browning, it's time to add the gravy ingredients and let it all simmer.
Once that is done you add back the veggies: the zucchini, winter squash and some corn and peas.
And here's a link to Alton Brown's recipe on Food Network.
I don't know what's going on with the spacing today. The spaces between paragraphs are invisible on my screen but show up on the preview and I can't seem to fix it. So sorry if it looks weird!