Grease a 9x13-inch dish with spray or butter.
Cook sausage in a large skillet over medium 8-10 minutes until browned and crumbled; drain grease.
Layer bread, sausage, and cheese in dish.
Whisk eggs, milk, mustard, salt, and pepper in a large bowl.
Pour evenly over filling; press gently with spatula to soak bread.
Cover tightly with plastic; refrigerate overnight or at least 4 hours.
Heat oven to 350°F; remove plastic and bake uncovered 45 minutes until golden and center set.
To freeze after step 5, cover tightly with plastic and foil; freeze up to 3 months. Thaw refrigerated overnight before baking as directed.