Preheat oven to 375 degrees farenheight (190 Celsius). Lightly grease 6 cup shallow baking dish.

In medium skillet, melt 1 tbsp of the butter over medium heat. Add onion and cook, stirring occasionally, until caramelized, about 8 minutes. Set aside

In medium saucepan, melt remaining 2 tbsp of butter over medium heat. Sprinkle in flour, salt pepper; cook stirring for 1 minute. Slowly stir in milk. Cook, stirring constantly until mixture thickens. Remove from heat and stir in cheese and mustard. Set aside

In prepared baking dish, layer half of the potato slices; top with half of the egg slices and half of the cheese sauce. Sprinkle with bacon and top with onions. Repeat with remaining potato, egg and sauce layers.

Topping:

In small bowl, toss crumbs with melted butter; sprinkle over casserole. Bake for about 20 minutes or until bubbly and golden. Serve warm