Bake pie crust according to directions on package. ( *note* I do this the night before to save steps in the morning). Preheat oven to 350 degrees. Spread mustard over bottom of baked pie shell. In a skillet, melt butter and saute onion, mushrooms and spinach. You can add bacon here if you wish. Spoon sauted mixture into pie crust. Top with cheese. In a large bowl, mix eggs, milk and salt and pour over vegetables and cheese. Sprinkle with paprika. Bake 40 to 45 minutes or until center is set..
I usually do the chopping of the mushrooms and onion and grating of the cheese the night before to save time in the morning.