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 1 tablespoon milk and 1 egg through food chute; process until dough forms a ball.
3. Divide dough into 24 equal portions, shaping each into a ball. Working with 1 portion at a time (cover remaining dough to prevent drying), roll each portion into a 2 1/2-inch circle on a surface lightly dusted with brown rice flour. Gently press dough circles into miniature muffin cups coated with cooking spray. Bake at 350° for 7 minutes or until very lightly browned. Remove from oven; cool on a wire rack.
4. To prepare filling, remove roots, outer leaves, and tops from leeks, leaving 2 inches of dark leaves. Cut leeks in half lengthwise; rinse with cold water. Cut each half crosswise into 1/4-inch-thick slices to measure 1/2 cup.
5. Heat a medium skillet over medium heat. Coat pan with cooking spray. Add leek and thyme; sauté 8 minutes or until tender and lightly browned. Cool.
6. Combine 1/2 cup milk and next 4 ingredients (through 2 eggs) in a medium bowl; stir with a whisk. Divide leek mixture and cheese evenly among crusts; spoon egg mixture over leeks and cheese. Sprinkle with freshly ground black pepper, if desired.
7. Place pans on a baking sheet. Bake at 350° for 20 minutes or until egg is set and crusts are lightly browned on edges. Cool 5 minutes in pan on a wire rack; remove from pan. Serve warm.