Crumble ground sausage into a medium skillet and cook over medium heat for 7–10 minutes, stirring occasionally, until fully browned. Drain excess fat and set aside.

In a medium bowl, whisk together beaten eggs, whole milk, mustard powder, and kosher salt. Stir in the cooked sausage and toasted bread cubes. Mix gently to coat. Add shredded Cheddar cheese and stir to combine.

Lightly grease a 9×13-inch baking dish. Pour the sausage, bread, and egg mixture into the dish, spreading it evenly. Cover and refrigerate for at least 8 hours or overnight.

Preheat the oven to 350°F (175°C). Bake the casserole, covered, for 45–60 minutes until the top begins to set and turn golden brown. Reduce the heat to 325°F (163°C) and bake uncovered for another 30 minutes. Check for doneness with a knife inserted in the center—it should come out clean.

Let the casserole cool for 5 minutes before slicing. Serve warm, garnished with optional sides like fresh fruit salad or hash browns for a complete meal.