Place croutons in bottom of prepared pan.
In a skillet, brown sausage over medium-high heat; drain.
Sprinkle cheese and sausage over croutons.
Blend eggs, milk and mustard together. Pour over ingredients in dish and refrigerate overnight.
Preheat oven to 300 degrees F.
Blend soup and milk; pour over all and bake for 1 1/2 hours.
Allow to set for 10 minutes before serving.