Line square baking dish, 8x8x2 inches, with aluminum foil, leaving 1 inch of foil overhanging at 2 opposite sides of pan; grease foil. Mix cornmeal and cold water in 2-quart saucepan. Stir in boiling water and salt. Cook about 5 minutes, stirring constantly, until mixture boils and thickens; reduce heat to low. Stir in onion, sage and 1/2 cup of the cheese. Cook uncovered 5 minutes, stirring occasionally; remove from heat.