Preheat oven to 400º F. Spray 13 x 9-inch baking dish with non stick cooking spray. Beat eggs, milk, sour cream, garlic salt, mustard and pepper in large bowl with whisk until blended. Set aside.
Place flour in a medium bowl. Cut in butter with pastry blender or fork until mixture resembles coarse crumbs. Add buttermilk. Stir just until dough leaves sides of bowl. Press into bottom of prepared dish with floured hands.
In a large bowl stir together potatoes, onions and cheeses into egg mixture until blended. Pour over dough. Spread out evenly. Bake for 35 to 40 minutes or until knife inserted in center comes out clean. Sprinkle with cooked bacon. Cool 10 minutes and serve