In small saucepan, melt 2 tablespoons butter. Add flour; cook 1 minute. Stir in milk. Cook until mixture boils and thickens; cool slightly.
In small skillet, melt remaining 1 tablespoon butter. Add eggs; cook and stir until soft scrambled. Remove from heat. Stir in milk mixture.
Spoon egg mixture over bread shell. Top with Canadian bacon, cheese, mushrooms and onions. Bake 20 minutes or until cheese is melted and bubbly.