Cut 2 sheets of thawed frozen puff pastry (one 17.25-ounce package) in half to form 4 rectangles.
Dividing evenly, top half of each rectangle with a mixture of 1½ cups shredded rotisserie chicken, 1½ cups grated Gruyere, and ½ cup frozen peas.
Seal the pastries and brush the tops with beaten egg.
Bake at 400 degrees F until golden, 20-25 minutes.
Serve with Dijon mustard.