Melt margarine in 23 cm pie or quiche pan. Add flour and powdered sugar, mix and make a dough. Form crust in pan. Place in 350 degrees oven for 2 to 3 minutes to set. Remove. Line crust with shrimp, mushrooms and onions. In a small bowl, combine sour cream, eggs, parsley flakes and cheese. Pour this mixture into pie crust. Return quiche to oven for 45 minutes. Note: May use canned crabmeat, diced ham or crumbled bacon in place of shrimp for variation.