Tips for shepherds pie
Just made this last night -- make sure to discard some of the fat that's rendered from browning the lamb (otherwise the base will be greasy). You'll need a little flour for thickening the mixture, and some good, dark stock (beef or veal). I always add a glug of wine to the meat and veggies to give them some more flavor, as well as a pinch of red pepper flakes.
Pre-cook and taste everything -- your mashed potatoes and your stew/meat mixture -- before assembling them in the casserole. If they taste great on their own, they'll be even better together!
A little cheese or roasted garlic added to the mashed potatoes for the topping will really dress it up. The best cheeses to use for this: cheddar, Gorgonzola, and Gruyère. Dust a bit of Parmesan over the top after it's baked and broil it for about 30 seconds; that will give you a beautiful golden crispy cheese top for the mashed potatoes.
Add roasted beets into your filling for extra color and veggies. These go best if you're filling the shepherd's pie with a sausage mixture. When I use this combination, I add the Gorgonzola to the potatoes. It turns out beautiful, with great flavor and presentation.
I recently made this Scottish Beef & Potato Pie: http://www.cooking.com/recipes-and-more/recipes/Scottish-Beef-and-Potato-Pie-recipe-2905.aspx
It's a great change from the traditional as it has a scalloped potato topping. I doubled the recipe using 1/2 ground beef 1/2 ground turkey. Omitted the egg. And, ended up pouring the cheese sauce over the potatoes rather than putting it on top of the meat. It was great!