Pour cream into a saucepan.

Place saucepan on low heat and reduce cream by a third.

While cream is reducing, melt butter in a small skillet.

Add minced green onions and sliced mushrooms to the skillet.

Season with salt, white pepper, and cayenne pepper.

Cook on medium-high heat for about ten minutes, stirring frequently.

Add white wine and reduced cream to the skillet.

Bring the mixture to a boil.

Remove from heat and allow to cool briefly.

Whisk in lightly beaten egg yolk.

Return the mixture to a boil, stirring constantly.

Remove immediately from heat and serve.