Preheat the oven to 180 degrees Celsius (356 degrees Fahrenheit).
Rinse the leek by holding it upside down under running water, while you separate the layers.
Cut the leek into thin slices.
Rinse the bell pepper and cut it into small cubes.
Mix leeks, bell pepper, cream cheese, and chicken together in a bowl.
Season with salt, pepper, garlic powder, onion powder, and lemon juice.
Roll out the puff pastry on a baking tray lined with parchment paper.
Spread the filling in an elongated shape in the center of the puff pastry.
Fold one side of the puff pastry over the middle of the filling, then fold the other side over.
Fold the ends of the puff pastry underneath, sealing them tightly.
Place the puff pastry in the oven and bake for 20-25 minutes until golden.
Remove the finished puff pastry from the oven and let it rest on the baking tray for a bit.
Cut the puff pastry roll into thick slices and serve.