Pick over the spinach and remove and discard any tough stems or blemished leaves. Rinse and drain the spinach well.
Bring enough water to the boil to cover the spinach when added. Add the spinach and cook, stirring occasionally, about 2 minutes. Drain thoroughly and press to extract any excess liquid. Put the spinach into the container of a food processor or electric blender and blend briefly.
Heat the butter in a saucepan and add the flour, stirring with a wire whisk. When blended and smooth, add one cup of the milk, stirring rapidly with the whisk. Stir in the cream and add the gruyere, salt, pepper, and nutmeg. Stir in the remaining one-half cup of milk.
Add the spinach and blend. There should be about three cups. Heat thoroughly and serve.
