These stuffed mushrooms are a classic party staple given a savory twist. The seasoning in Italian sausage makes them especially flavorful, and using hot Italian sausage really gives the stuffing zing. Italian turkey sausage can be substituted, however, use only Parmigiano Reggiano cheese for the tastiest results (and use 24-month-old Parmigiano Reggiano for optimum flavor). The stuffing can be made and the mushrooms filled in advance; refrigerate them until ready to bake.