I tried a few variations of this crust--including one with mashed sweet potato mixed with cauliflower and egg, like the faux pizza crust people love these days--but this is my favorite version. Be sure to overlap the potato slices enough to "seal" the base, but don't worry if a bit of filling leaks through, it won't be the end of the world! —Posie (Harwood) Brien
Peel the sweet potatoes and cut them very thinly (a mandolin is useful here). Lightly grease a 9" pie plate with olive oil and layer the slices over the base and up the sides of the plate (cutting some slices in half will help here). Try to make sure the slices overlap everywhere to avoid leakage.
Bake the "crust" for 20 minutes, then remove from the oven and turn the heat up to 375°F.
Meanwhile make the filling: whisk together the eggs, milk, and cream. Whisk in the salt, pepper, and mustard and stir in the greens and 3/4 cup of the cheese, reserving 1/4 cup of cheese.
Sprinkle the reserved 1/4 cup of cheese over the sweet potato crust, then pour the filling on top.
Bake for 35 minutes, or until the top of the quiche is just set and starting to turn a light golden brown. Remove from the oven and let cool a bit before slicing.