Chop sage leaves. In a 3 to 4 quart heavy saucepan bring water with sage and salt to a boil and gradually add cornmeal in a thin stream, whisking. Cook polenta over moderate heat, whisking, 2 minutes.
Gently simmer polenta, covered, stirring for 1 minute after every 10 minutes of cooking, about 20 minutes total. Stir in Parmesan. Remove pan from heat and cover to keep warm. Polenta will keep warm, covered, about 20 minutes. Stir polenta before serving. Makes about 5 cups.