Prepare the Bechamel sauce. Melt the butter in a deep frying pan or saucepan, add the flour and fry it over medium heat, stirring with a spatula, until slightly brownish. Add cream, milk, tarragon, reduce the heat to a minimum and cook the sauce until it acquires the consistency of medium-fat sour cream. At the end, add salt, white pepper and grated nutmeg and mix.