Cook onion in butter till tender but not brown. Stir in flour. Add milk all at once. Cook and stir until thickened and bubbly. Cook and stir 1 to 2 minutes more. Add cheeses; stir till melted. Place half of the sliced eggs in the bottom of a 10x6x2 inch baking dish; pour half of the cheese sauce over. Sprinkle with half of the crumbled bacon and half of the crushed potato chips. Repeat layers. Bake in a 350F oven for 15 to 20 minutes.
Makes 6 servings