Butter a 13x9" casserole dish. Place bread in dish, sprinkle cheese on top. Sprinkle sausage on top next, then top that with about 1 c. fried onion rings (canned).
Beat eggs and 2 1/2 c. milk with dry mustard and cayenne pepper. Pour mixture over casserole and refrigerate overnight.
Next morning, mix soup with remaining 1/2 cup milk and pour over casserole. Bake at 350F., about 50 minutes. Serve immediately.