Heat a large skillet over medium-high heat.
Cook and stir the sausage in the hot skillet until crumbly, evenly browned, and no longer pink.
Drain and discard any excess grease.
Grease a 9x13-inch baking dish.
Spread the bread cubes into the prepared dish.
Sprinkle the browned sausage and cheddar cheese overtop.
Beat the eggs in a mixing bowl until smooth.
Add the salt, mustard powder, and 2 cups of milk.
Whisk until evenly blended.
Pour the egg mixture over the bread cubes. Cover and refrigerate overnight.
Preheat an oven to 300 degrees.
Whisk together the cream of mushroom soup and 1/2 cup milk.
Pour over the casserole.
Bake in the preheated oven until firm and golden brown, about 50 minutes.