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. Drain off any excess grease thoroughly and set the sausage aside to cool slightly.

Assemble the Base Layers: Scatter the cubed bread evenly across the bottom of the prepared dish. Sprinkle the cooked and drained sausage evenly over the layer of bread cubes. Then, 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 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 until the seasonings are thoroughly incorporated.

Saturate and Chill (Crucial Step): Slowly and evenly pour the egg and milk mixture over the bread, sausage, and cheese layers, ensuring all the bread cubes are moistened. Press down gently to help the bread absorb the liquid. Cover the baking dish tightly and refrigerate for a minimum of 3 hours, or ideally, overnight (8 to 12 hours).

Preheat the Oven: About 30 minutes before baking, remove the casserole from the refrigerator. Preheat your oven to 350°F (175°C).

Bake the Casserole: Remove the cover/foil from the dish. Place the casserole in the preheated oven and bake for 45 to 55 minutes. The casserole is finished when the center is set (no longer jiggly) and a knife inserted near the center comes out clean. If the top browns too quickly, loosely cover the dish with aluminum foil for the remaining 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.