In a heavy saucepan, combine the stock and cream, and bring to a boil. Add the nutmeg and salt. Whisk in the polenta and semolina and cook over low heat for about 20 to 30 minutes, stirring often, until the grains are soft. Fold in the cheeses. Serve immediately or reserve.
To encourage polenta to come cleanly out of the pan, cook over medium heat. Run a spatula or wooden spoon around the sides of the pan to clean off the polenta. Do not stir, but wait and watch for a few seconds until a large bubble begins to form and pushes the polenta upward. Pour immediately into a warm dish.
The polenta can be made ahead and reheated: add 1/4 to 1/2 cup water or stock, cover the dish, and reheat in the microwave or over low heat. Whisk well before serving. Grate a dusting of Parmesan over the top just before serving.