Taste testing by family and friends is an integral part of the cookbook-writing process. When I served this quiche with its buttery crust, fluffy egg center with smoky and salty bacon, and blend of two cheeses, I thought I had a hit. When my brother-in-law, Kevin, went back for thirds and asked for more, I knew I was right.
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 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 egg through food chute; process until dough forms a ball.
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.
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.
Layer spinach, bacon, and cheeses in crust. Combine milk, pepper, salt, eggs, and egg whites in a medium bowl; stir well with a whisk. Pour over layered ingredients in crust.
Bake at 350° for 35 minutes or until crust is golden brown and eggs are set. Let stand 5 minutes before slicing.