Heat oven to 400° F. In a medium bowl, combine the chicken, Gruyère, and peas.
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.