Prepare the Pan: Grease a standard 9x13 inch baking dish thoroughly with cooking spray or butter. Set aside.

Cook the Sausage: Place the bulk breakfast sausage in a large skillet over medium-high heat. Cook, breaking it up with a spoon, until it is completely browned and cooked through. Drain off any excess grease and set the sausage aside to cool slightly.

Assemble the Base Layers: Scatter the cubed bread evenly across the bottom of the prepared 9x13 inch baking dish. Sprinkle the cooked and drained sausage evenly over the layer of bread cubes.

Add the Cheese: Evenly sprinkle the 2 cups of shredded cheddar cheese over the sausage and bread layers.

Prepare the Egg Custard: In a large bowl, whisk the 10 large eggs vigorously until they are well combined and slightly frothy. Pour in the 2 cups of milk. Add the salt, black pepper, dry mustard powder (if using), and garlic powder (if using). Continue whisking the mixture until all the seasonings are thoroughly incorporated.

Saturate the Casserole: Slowly and evenly pour the egg and milk mixture over the bread, sausage, and cheese layers in the baking dish, ensuring all the bread cubes are moistened. Press down gently on the top layer with a spatula or the back of a spoon to help the bread absorb the liquid.

Chill and Soak (Crucial Step): Cover the baking dish tightly with plastic wrap or aluminum foil. Refrigerate the casserole for a minimum of 3 hours, or ideally, overnight (8 to 12 hours). This allows the bread to fully soak up the egg mixture, preventing a dry casserole.

Preheat the Oven: About 30 minutes before you plan to bake, remove the casserole from the refrigerator to allow it to come slightly closer to room temperature. Preheat your oven to 350°F (175°C).

Bake the Casserole: Remove the plastic wrap or foil from the dish. Place the casserole in the preheated oven and bake for 45 to 55 minutes.

Check for Doneness: The casserole is finished when the center is set (no longer jiggly), the edges are golden brown, and a knife inserted near the center comes out clean. If the top begins to brown too quickly, loosely cover the dish with aluminum foil for the last 10 to 15 minutes of baking time.

Rest and Serve: Remove the finished casserole from the oven and allow it to rest for 5 to 10 minutes before slicing and serving. This resting period helps the casserole set completely and makes serving much easier.