In a large skillet, brown sausage- drain and crumble.
Butter a 9 x 19 Pyrex dish.
Place bread (torn into pieces) in bottom.
Cover with sausage.
Cover with cheese.
In a large bowl, beat eggs; mix in milk, salt and mustard.
Blend well and pour over casserole.
Cover and refrigerate overnight.
The next morning, bake uncovered at 350 for 45 minutes.
Slice into squares and serve hot.