We develop our recipes using a 4-to 5-ounce skinless, boneless chicken breast half per serving. However, there are a range of sizes available in-store, from the butcher counter to the meat case and the freezer section. Use whichever you prefer; just follow the recipe as written above for the best result. Larger chicken breasts may require a little longer cooking time.
Preheat the oven to 400 degrees F.
Beat the egg and water in a small bowl with a fork.
Sprinkle the chicken with the thyme and black pepper. Heat 1 tablespoon of the butter in a 10-inch skillet over medium-high heat. Add the chicken and cook until well browned on both sides, 10 minutes. Remove the chicken from the skillet. Cover and refrigerate for 15 minutes.
Heat the remaining 1 tablespoon butter in the skillet. Add the mushrooms and onions and cook until tender and the liquid evaporates, stirring occasionally. Stir in the parsley.
Stir the cream cheese and mustard in a small bowl.
Sprinkle the work surface with the flour. Unfold the pastry on the work surface. Roll the pastry sheet into a 14-inch square. Cut into 4 7-inch squares.
Spoon about 1 tablespoon of the mushroom mixture on each square. Top with the chicken. Spread about 1 tablespoon cream cheese mixture on the chicken. Brush the edges of the pastry with the egg mixture. Fold the corners of the pastries up over the chicken and press to seal. Place the pastries seam-side down onto a baking sheet. Brush with the egg mixture.
Bake until the pastries are golden brown, 25 minutes.