PREPARATION:

1. Sprinkle chicken with 1/4 teaspoon salt and 1/8 teaspoon pepper. Melt 2 tablespoons butter in large skillet over medium heat. Brown chicken breasts in batches 6 minutes, turning once. Transfer to plate.
2. Melt remaining 2 tablespoons butter in skillet over medium heat. Add mushrooms and shallots. Cook and stir 5 minutes or until mushrooms release their liquid. Add wine, thyme, remaining 1/2 teaspoon salt and 1/8 teaspoon pepper; simmer , stirring often. Cool.
3. Roll out each pastry sheet to 15X12-inch rectangle. Cut each into 3 (12X5-inch) rectangles. Cut small amount of pastry from corners into decorative shapes; set aside. Brush beaten egg white over pastry rectangles. Place 1 cooled chicken breast on one side of each pastry rectangle. Spread 1/2 teaspoon mustard over each chicken breast, then spread with 1/4 cup mushroom mixture. Fold pastry over chicken; press edges to seal. Place on ungreased baking sheet; top with pastry shapes. Whisk egg yolk and milk in small bowl; brush over pastry. Cover loosely with plastic wrap. Refrigerate until cold, 1 to 4 hours.
4. Preheat oven to 400°F. Bake chicken 25 to 30 minutes or until chicken is 170°F pastry is deep golden brown.