Preheat the Oven: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
Prepare the Filling: In a mixing bowl, combine the diced chicken, béchamel sauce, onion, garlic, cheese, and herbs. Season with salt and pepper. Set aside.
Prepare the Pastry: Roll out the puff pastry on a lightly floured surface. Cut into squares or rectangles (depending on your preferred size).
Assemble the Friands: Spoon a small amount of the chicken mixture into the center of each pastry square. Fold the pastry over the filling to form a pocket or triangle. Seal the edges by pressing with your fingers or a fork.
Egg Wash: Beat the egg and brush it over the top of each friand for a golden finish.
Bake: Place the friands on the prepared baking sheet and bake for 15-20 minutes, or until the pastry is golden and puffed.
Cool and Serve: Let the friands rest for a few minutes before serving.