Prepare the Pan & Sausage: Grease a standard 9x13 inch baking dish thoroughly. In a large skillet, cook the bulk breakfast sausage over medium-high heat until completely browned. Drain off any excess grease and set aside to cool slightly.

Assemble the Base Layers: Scatter the cubed bread evenly across the bottom of the prepared baking dish. Sprinkle the cooked and drained sausage evenly over the bread, followed by the 2 cups of shredded cheddar cheese.

Prepare the Egg Custard: In a large bowl, whisk the 10 large eggs vigorously until slightly frothy. Whisk in the 2 cups of milk (or half-and-half). Add the salt, black pepper, dry mustard powder (if using), and garlic powder (if using). Whisk until thoroughly combined.

Saturate and Chill: 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 absorption. Cover the dish tightly with plastic wrap or aluminum foil 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 plastic wrap or foil. Place the casserole in the preheated oven and bake for 45 to 55 minutes. If the top begins to brown too quickly, loosely cover the dish with aluminum foil for the last 10 to 15 minutes.

Check for Doneness: The casserole is finished when the center is set, the edges are golden brown, and a knife inserted near the center comes out clean.

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 ensures the casserole sets completely.