Cook the pork sausage in a large skillet over medium-high heat for approximately 10 minutes, breaking it into crumbles as it cooks. Continue until the meat is evenly browned and no pink remains. Drain thoroughly and discard the excess grease.
Crack the eggs into a large mixing bowl and beat them until blended. Add the milk, salt, and dry mustard, then stir until the seasonings are evenly distributed throughout the egg mixture.
Add the cubed white bread, shredded Cheddar cheese, and drained cooked sausage to the egg mixture. Fold everything together until the ingredients are evenly incorporated.
Coat a 7 × 11-inch baking dish lightly with nonstick cooking spray. Transfer the casserole mixture to the prepared dish and spread it into an even layer.
Cover the baking dish and place it in the refrigerator for 8 hours or overnight, giving the bread sufficient time to soak up the liquid mixture.
Preheat the oven to 350°F (175°C). Take the refrigerated casserole out 30 minutes before baking and allow it to stand at room temperature.
Place the casserole in the preheated oven and bake for approximately 40 minutes. It is finished when a knife inserted near the center comes out clean.
Remove the baked casserole from the oven and allow it to cool for a few minutes before serving hot.