Brown and drain the sausage. Drain the mushrooms, cube bread. In large bow, combine milk, eggs, mustard, onion, grated cheese, soup and Worcestershire sauce. Add sausage and mushrooms. Generously butter 9 x 13 x 2 inch casserole. Pour mixture, cover and refrigerate overnight. Bake, uncovered, at 300ºF. for 1 1/2 hours.