Preheat oven to 425. Bake pie shell for 10 minutes; remove and let cool. Reduce oven heat to 350.`
Saute mushrooms in butter over medium high heat til tender. Remove from pan with slotted spoon and set aside.
In bowl toss cheese with flour; add eggs, milk, parsley, garlic salt and pepper. Stir in mushrooms. Pour into crust. Bake for 1 hour or til set, covering edges with foil to prevent over browning. Let stand for 10 minutes before cutting.