Spread croutons in an even layer in a 3-quart rectangular baking dish. Spread sausage evenly over croutons. Sprinkle spinach and carrot evenly over sausage.
In a medium bowl beat eggs with a whisk; whisk in milk, mushroom soup, mushrooms, the 1 cup cheddar cheese, the 1 cup Monterey Jack cheese, and the dry mustard until well mixed. Pour over layers in dish. Cover and chill for at least 8 hours or up to 24 hours.
Preheat oven to 325 degree F. Uncover casserole and bake in preheated oven for 45 minutes. If desired, sprinkle with additional cheese. Bake about 10 minutes more or until edges are bubbly and center is heated through. Let stand for 10 minutes before serving. Makes 12 servings.