1. Preheat oven to 350°.
2. To prepare crust, weigh or lightly spoon flours and potato starch into dry measuring cups; level with a knife. Place flours, potato starch, xanthan gum, and 1/4 teaspoon salt in a food processor; pulse 2 times or until blended. Add butter; process 30 seconds or until mixture resembles coarse meal. With processor on, slowly add milk and 1 egg through food chute; process until dough forms a ball.
3. Press mixture gently into a 4-inch circle on plastic wrap; cover. Chill 30 minutes. Slightly overlap 2 sheets of plastic wrap on work surface. Unwrap and place chilled dough on plastic wrap. Cover dough with 2 additional sheets of overlapping plastic wrap. Roll dough, still covered, into a 12-inch circle. Remove top sheets of plastic wrap; fit dough, plastic wrap side up, into a 9-inch pie plate coated with cooking spray. Remove remaining plastic wrap. Fold edges under; flute. Pierce bottom and sides of dough with a fork. Bake at 350° for 10 minutes. Cool on a wire rack.
4. To prepare filling, cook bacon in a large nonstick skillet over medium heat 8 minutes or until crisp. Remove bacon from pan; crumble. Add spinach to drippings in pan; sauté 1 minute or until spinach wilts.
5. Layer spinach, bacon, and cheeses in crust. Combine milk, pepper, 1/8 teaspoon salt, eggs, and egg whites in a medium bowl; stir well with a whisk. Pour over layered ingredients in crust.
6. Bake at 350° for 35 minutes or until crust is golden brown and eggs are set. Let stand 5 minutes before slicing.