In a large skillet, cook sausage over medium heat until no longer pink; drain. Place bread cubes in a greased 13-in. x 9-in. baking dish. Layer with mushrooms, onions and sausage.
In a large bowl, whisk the eggs, cream, mustard and salt. Pour over top; sprinkle with cheese. Cover and refrigerate overnight.
Remove from the refrigerator 30 minutes before baking. Bake, uncovered, at 350° for 60-65 minutes or until a knife inserted near the center comes out clean. (Cover loosely with foil if top browns too quickly.) Let stand for 10 minutes before serving.
Serve immediately or before refrigerating and baking, cover and freeze casserole for up to 3 months.
To use frozen casserole: Thaw in the refrigerator overnight. Remove from the refrigerator 30 minutes before baking. Bake according to directions. Yield: 12 servings.