Combine first 4 ingredients; cut in shortening until crumbly. Stir in sour cream. Cover; refrigerate 30 minutes. Press pastry into a 9 inch pie plate. Bake at 375° F for 10 minutes. Cool completely.
Combine mayonnaise, Cheddar cheese, mushrooms, bacon, green pepper and onion. Layer half of the tomatoes in crust; top with half of the mayonnaise mixture. Repeat layers. Bake at 350° F for 30 to 35 minutes or until golden brown. Refrigerate leftovers.
*Reduced-fat or fat-free sour cream and mayonnaise may not be substituted.Servings: 6 to 8