How to Make It
To prepare crust, weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, 1 1/2 teaspoons sugar, baking soda, and xanthan gum in a medium bowl; stir with a whisk. Cut in butter and shortening with a pastry blender or 2 knives until mixture resembles coarse meal. Add 9 tablespoons 1% low-fat milk; toss with a fork until moist. Gently press mixture into a 4-inch circle on 2 sheets of heavy-duty plastic wrap that overlap; cover with additional overlapping plastic wrap. Chill 30 minutes.
Roll dough, still covered, into a 12-inch circle. Fit dough, plastic wrap side up, into a 9-inch pie plate coated with cooking spray. Remove remaining plastic wrap. Press dough against bottom and sides of pan. Fold edges under or flute decoratively. Chill until ready to fill.
Preheat oven to 400°.
While crust chills, pierce potatoes with a fork; bake at 400° for 1 hour or until tender. Cool. Peel potatoes; mash.
Combine mashed potato, 3/4 cup sugar, and next 6 ingredients (through eggs) in a large mixing bowl; beat with a mixer at medium speed until well blended. Slowly stir in evaporated milk. Pour filling into prepared crust. Bake at 400° for 15 minutes. Reduce oven temperature to 350° (do not remove pie from oven); bake an additional 45 minutes or until center is set. Cool completely. Serve with whipped topping, and sprinkle with nutmeg, if desired.