Cook onion in butter till tender, but not brown; blend in flour.
Cook, stirring constantly, till mixture thickens and bubbles.
Add cheese; stir till melted.
Place a layer of egg slices in 10 x 6 x1 1/2-inch baking dish.
Cover with half the cheese sauce, half the potato chips, and half the bacon.
Bake in moderate oven (350°) for 15 to 20 minutes or till hot through