1. Heat oven to 400° F. In a medium bowl, combine the chicken, Gruyère, and peas.
2. Cut the 2 sheets of puff pastry in half to form 4 rectangles and place on a baking sheet. Dividing evenly, top half of each rectangle with the chicken mixture. Fold over and seal the turnovers; brush the tops with the egg. Bake until golden, 20 to 25 minutes. Serve with the mustard.
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