Prepare the Sausage: Crumble the breakfast sausage into a large skillet. Cook over medium heat until thoroughly browned, ensuring no pink remains. Drain off any excess grease completely and set the sausage aside to cool slightly.

Prepare the Baking Dish: Preheat your oven to 350°F (175°C). Lightly grease a standard 9x13 inch casserole dish with butter or non-stick cooking spray.

Arrange the Bread and Fillings: Distribute the cubed bread evenly across the bottom of the prepared dish. Sprinkle the cooked and drained sausage evenly over the bread, followed by the 2 cups of shredded cheese.

Whisk the Custard: In a large bowl, whisk the 8 large eggs vigorously until fully combined. Pour in the 2 cups of milk and add the dry ground mustard, salt, and black pepper. Continue whisking until all seasonings are incorporated.

Assembly and Soaking (Crucial Step): Carefully pour the entire egg and milk custard mixture evenly over the sausage, bread, and cheese layers. Gently press down on the bread cubes with a spatula to ensure they fully absorb the liquid and become saturated.

Refrigerate: Cover the casserole dish tightly with plastic wrap or aluminum foil. Place the casserole in the refrigerator and allow it to soak for a minimum of 4 hours, or ideally, overnight (up to 12 hours).

Bake Preparation: Remove the casserole from the refrigerator about 30 minutes before baking time to allow it to temper slightly. Remove the plastic wrap.

Bake the Casserole: Place the uncovered casserole dish into the preheated 350°F (175°C) oven. Bake for 50 to 60 minutes. The casserole is done when the center is set, the eggs are no longer jiggly, and the top is puffed and lightly golden brown.

Rest and Serve: Remove the casserole from the oven and allow it to rest for 5 to 10 minutes before cutting into squares and serving immediately.