Poke holes on the inside of the pie crust with a fork. Line the inside of the pie crust with foil and bake at 450 degrees for 8 minutes. Remove the foil and bake for an additional 5 minutes or until the crust is set and dry. Remove the crust and reduce the oven to 350 degrees.
Sauté the onions, peppers, and mushrooms in the butter until softened.
Combine the vegetables with the remaining ingredients and pour the mixture into the pie crust.
Bake the quiche at 350 degrees for 40 minutes or until the center is set. Let stand for 10 minutes before cutting.