Preparation

• Melt the butter in a saucepan. Whisk in the flour. Add the milk and whisk until sauce is thick.
  Mix in egg yolk and cook over low heat for 2 minutes. Add cheese. Stir over low heat until melted. Remove sauce from heat. Season with salt and pepper and nutmeg.
  Pour the sauce in a container and cover with buttered wax paper. Heat before serving.