Bechamel sauce is a foundation sauce of French cuisine and is the basis for countless dishes. It may be made with milk alone or enriched with additional cream, or part of liquid may be stock. Melt butter in saucepan and stir in flour. Cook, stirring with wire whisk, until thoroughly blended. Scald milk with onion and parsley. Strain into butter/flour mixture, stirring constantly with wire whisk. Blend in Gruyere. When mixture is thickened and smooth, add half and half. Season with nutmeg; salt and pepper to taste. Add mustard and Worcestershire sauce to taste.