Oh, boy. This is an easy one. It's not even really a pie and really should be called shepherd's casserole or something.
The first thing you do is preheat your oven to 400°F/200°C and make some gravy (there are some GF gravy tips at that link). Once your gravy is done, get yourself some veggies (I use mixed frozen veggies when I am short on time) and steam or boil them until tender (if you don't, this will increase the baking time). To make the 'pie' a little more filling, you can add some TVP or seitan (seitan is not GF) or even some cooked lentils or some small kind of cooked bean, although I prefer to add cooked buckwheat. Mix the veggies and the other filling into the gravy and pour into a small casserole dish.
|Pasta is not a typical shepherd's pie side dish. ;p|
Now you need to make some mashed potatoes. I linked you up to an old recipe of mine, but make them however you like. Feel free to use instant if you are lazy or in a hurry. Anyway, when the mashed potatoes are ready, spread that mash across the top of the gravy-covered stuff in the casserole dish.
If you haven't pre-cooked the vegetables, cover the dish and bake until bubbly, uncover and bake a further 10-15 minutes. If you have pre-cooked the veggies, you just need to bake everything for about 10-15 minutes. The top of the mashed potatoes should just be starting to brown up and the gravy should be bubbling up around the sides of the dish.