Sauce Duxelles (Béchamel, Onions, Mushrooms, Sherry, Crème Fraîche, Egg Yolk)

This 2nd generation mother sauce derives from a finished Sauce Béchamel. Serve with poultry, eggs and vegetables.

Sauce Béchamel:
4 Tbsp. butter
4 Tbsp. all-purpose flour, unbleached
2½ cups milk, warmed but not hot
1 pinch nutmeg, freshly grated
¼ tsp. sea salt
⅛ tsp. white pepper, freshly ground

Sauce Duxelles:
2 onions, minced
1 cup mushrooms, minced
1 Tbsp. sherry
¼ cup crème fraîche mixed with 1 egg yolk

Melt the butter in a heavy-bottomed saucepan, add the onions and sauté for 8 minutes. Add the mushrooms and sauté another 5 minutes. And teh sherry and stir in the flour and cook, whisking constantly, until the paste cooks and bubbles a bit, but don't let it brown - about 2 minutes.

Add the warm milk gradually, continuing to whisk as the sauce thickens. Bring it to a boil. Add nutmeg, salt and pepper.

Stir in a blended mixture of crème fraîche and egg yolk. Lower the heat to simmer, and cook, stirring for 2-3 minutes more. Remove from the heat.

Makes about 2 cups.