In a 5-quart Dutch oven, combine cold water and salt. With wire whisk, gradually beat in cornmeal until smooth. Whisk in boiling water. Heat to boiling over high heat. Reduce heat to medium-low and cook until mixture is very thick, 20 to 25 minutes, stirring frequently with wooden spoon.
Stir margarine and grated Parmesan, if using, into polenta until margarine has melted. Serve immediately.
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