Dilute the egg yolks in the cold fond blancand in the cooked mushrooms reduction, then put the mixture in a saucepan and season with salt, pepper, a pinch of nutmeg and a few drops of lemon juice.
Over the heat whisk and add the veloute sauce.
Continue to skim and add, a bit by bit, 1/2 cup of cream.
Once the sauce coats the spatula, remove from the heat and pass through a cloth sieve. Keep the sauce in a bain-marie with a little butter on the surface. When ready to use it, fold in the remaining cream and the butter.