Heat the Velouté sauce, preferably in a double boiler. Mix the egg yolk and cream, and stir into the sauce, cook without boiling, till the egg yolk thickens. Whisk in the butter, a small pat at a time. Add a pinch of nutmeg, a few drops of lemon juice, season and use the sauce at once.
Serve with any meat, poultry, fish or vegetables.