Cook, stirring, until the mushrooms are softened and lightly browned, 2 to 3 minutes. Add:
1/3 cup sweet or dry sherry
Boil until the sherry is nearly evaporated, about 1 minute. Add:
1 cup heavy cream
1/2 cup chicken stock
Boil until the sauce is thick enough to lightly coat a spoon, about 5 minutes. Add any accumulated chicken juices and reduce again until thick. Stir in:
2 tablespoons finely chopped fresh parsley
Pinch of freshly grated or ground nutmeg or ground mace
Salt and ground white pepper or ground black pepper to taste
Season with:
Several drops of fresh lemon juice
Spoon the sauce over the chicken and serve immediately.