Brown sausage in a large skillet over medium, crumbling it; drain and set aside.
Grease a 9x13-inch dish; layer bread evenly in bottom.
Sprinkle sausage and cheese over bread.
Whisk eggs, milk, mustard, salt, and pepper in a large bowl until combined.
Pour evenly over bread, sausage, and cheese; press gently to moisten all bread.
Cover with plastic wrap; refrigerate overnight or at least 8 hours.
Remove from refrigerator 20 minutes before baking; preheat oven to 350°F.
Bake uncovered 45 minutes or until top is golden brown and center set.