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