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.
Cool.
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.
Brush decorations.
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.