Place butter in an 11x17x2-inch baking dish. Place dish in microwave oven to melt, tilting to coat dish. Pour croutons over melted butter; sprinkle cheese over croutons and set aside.

Combine milk, eggs and mustard. Mix well. Pour egg mixture over cheese and sprinkle with crumbled bacon. Bake at 325° F for 40 to 50 minutes or until set. Let casserole stand at room temperature 5 to 10 minutes before serving. Yield: 6 to 8 servings.