Crumble the sausage in a medium skillet, and cook over medium heat until browned. Drain the fat. Combine egg, milk, salt, mustard powder in a medium bowl. Add the chesse, sausage and bread cubes, and stir to coat. Pour mixture into a greased 9x13 inch baking dish. Cover, and refrigerate for 8 hours or over night. Preheat oven to 350F (175C). Cover, and bake for 45 to 60 minutes. Reduce heat to 325F (165C) and bake for an additional 30 minutes or until set.