To make a scrambled egg breakfast casserole with bacon and cheddar cheese, cook 1 14-ounce package of bacon, and then crumble it. Set a large frying pan over medium-high heat, and add 5 tablespoons of butter. When the butter melts, add the crumbled bacon and 2/3 cup of chopped onions. Cook the onions and bacon in the butter until the onions are soft, about five to 10 minutes.
Beat 18 eggs, and add them to the onions and bacon. Continuously stir the egg mixture as it cooks until the eggs begin to set but are still slightly wet, approximately five minutes.
In a separate pan, add 5 tablespoons of butter. Melt the butter over medium-high heat, and mix in 5 tablespoons of all-purpose flour until thoroughly combined. Pour in 3 cups of milk, and mix in 1 1/2 cups of Cheddar cheese, 2/3 teaspoon of salt and 1/4 teaspoon of ground black pepper. Stir the ingredients continuously until the cheese completely melts, approximately five to seven minutes.
In a mixing bowl, combine 2 1/2 cups of seasoned stuffing mix with 1/4 cup of melted butter. In a separate bowl, combine the eggs and melted cheese mixture. Transfer the eggs and cheese mixture to a greased 9-by-14-inch baking pan, and top it with the stuffing mix. Cover the pan, and allow it to set overnight in the refrigerator.
Heat an oven to 350 degrees Fahrenheit. Uncover the pan, and bake it for 30 minutes or until hot in the center.