Slow-cooking the polenta in a double boiler gives it a light but creamy texture without needing to stir or whisk. If using instant polenta or grits, decrease cooking time and follow package directions.
Heat 1 1/2 inches water in a large saucepan until simmering. Combine milk, salt, and nutmeg in a large metal mixing bowl, and place over simmering water until mixture is hot.
Stir polenta into hot milk mixture, whisking until blended. Cover with foil, and cook over medium-low heat 1 1/2 hours. Uncover and stir in butter and Parmesan cheese.