Lightly butter a 9-x-13-inch casserole dish or 10” cake pan insert for Dutch oven cooking.
In a large bowl, whisk together the eggs and 2 1/2 cups of the milk. Whisk in the mayonnaise, mustard, salt, and pepper.
Spread the croutons on the bottom of the casserole dish. Sprinkle the cheese and sausage evenly over the croutons. Pour the egg mixture over the casserole.
In a small bowl, whisk together the remaining 1/2 cup milk and the soup concentrate. Spoon heaping teaspoonfuls of the soup mixture evenly on top of the casserole. Cover and place in the icebox for at least 6 hours or overnight.
Rise and shine, it’s sausage time! Preheat the oven to 350°F.
Uncover and cook the casserole for 11/2 hours, or until it is bubbling and heated through. Serve warm.