|Boneless skinless chicken breast halves||36 Ounce (About 6 Pieces, 6 Ounces Each)|
|Salt||3⁄4 Teaspoon (Divided)|
|Ground black pepper||3⁄4 Teaspoon (Divided)|
|Butter/Margarine||1⁄4 Cup (4 tbs) (Divided)|
|Mushrooms||12 Ounce, chopped (Button/Crimini)|
|Chopped shallots/Onion||1⁄2 Cup (8 tbs)|
|Port wine/Cognac||2 Tablespoon|
|Thyme leaves/1 teaspoon dried thyme leaves, crushed||1 Tablespoon|
|Frozen puff pastry||17 1⁄4 Ounce (1 Package, Thawed)|
|Egg||1 , separated|
|Dijon mustard||1 Tablespoon|
Sprinkle chicken with 14 teaspoon salt and 1/8 teaspoon pepper.
Melt 2 tablespoons butter in skillet over medium heat.
Cook 3 chicken breasts 6 minutes or until golden brown, turning once.
Transfer to plate.
Cook remaining chicken; cool slightly.
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 10 minutes or until liquid evaporates, stirring often.
Roll out each pastry sheet to 15x12-inch rectangle.
Cut each into 3 (12x5-inch) rectangles.
Cut small amount of pastry from corners to use as decoration, if desired.
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.
Fold edge of bottom dough over top; press edges to seal.
Place on ungreased baking sheet.
Brush combined egg yolk and milk over pastry; top with pastry scraps cut into decorative shapes, if desired.
Cover loosely with plastic wrap.
Refrigerate until cold, 1 to 4 hours.
Preheat oven to 400°F.
Remove plastic wrap.
Bake chicken 25 to 30 minutes or until deep golden brown and chicken is 160°F.
Garnish, if desired.