Let puff pastry thaw while you make the filling. Preheat oven to 400º F.
Sprinkle Chicken With Thyme, seasoning with Salt And Pepper to taste. Melt 1 Tbsp. Butter In Skillet. Add Chicken And Cook Until Browned. Remove Chicken From Skillet. Cover And Refrigerate 15 Min. Or Up To 24 Hr.
Add Remaining Butter To Skillet. Add Mushrooms And Onion And Cook Until Tender And Liquid Is Evaporated. Stir In Parsley.
In a small bowl, Mix Cream Cheese and Mustard until combined. Set Aside.
Unfold pastry on lightly floured surface.
Roll into 14" square and cut into 4 (7") squares. Spoon about 1 tbsp.
Mushroom mixture on each square. Top with chicken. Spread 1 heaping tbsp. Cream
cheese mixture on chicken. Brush edges of squares with egg mixture. Fold each
corner to center on top of chicken and seal edges. Place seam-side down on
baking sheet. Brush with egg mixture