Step 1

Cook puff pastry according to package directions.

Step 2

Meanwhile, in a large saucepan, melt half of the butter, stir in flour with a wooden spoon until a large ball forms. Add the cold milk all at once and bring to a boil, stirring constantly until the sauce thickens. Season with nutmeg, salt and pepper.

Step 3

Add the egg yolk.  While stirring continuously, add the cream, then the grated imported Gruyère AOP.

Step 4

Cut or shred chicken into small pieces. In a deep sided pan, saute the chicken and mushrooms in the other half of butter over low heat.  Spoon the chicken mixture into the sauce, add lemon juice and heat everything through.

Step 5

Remove pastry from oven and fill generously with mixture.  Sprinkle with parsley, serve immediately.

Step 6

Serves 4