In a large skillet, cook sausage and mushrooms over medium heat until meat is no longer pink, drain. 

Place half of the croutons in a greased 9×13” baking dish; top with 2 cups of sausage mixture and then half of the cheese.  Repeat layers. 

In a large bowl, combine eggs, milk, mustard, salt, and pepper; pour over bread mixture. 

Bake, uncovered, 350 degrees for 50-60 minutes or until a knife inserted near the center comes out clean.  Let stand for 10 minutes before serving.