Preheat & Prep. Preheat oven to 400 degrees F. Line two baking sheets with parchment paper or lightly grease them and set aside.
Combine Filling. In a large bowl, combine chicken, spinach, red pepepr, cream cheese, gruyere, basil, garlic powder, salt, and pepper. Toss until well combined and cream cheese is completely mixed in.
Make Egg Wash. In a small bowl, combine an egg wash by beating egg with water.
Assemble Puffs. Roll out puff pastry on a sheet of parchment paper or a lightly floured surface. Slice each sheet into 4 pieces (for 8 squares total). Divide filling among the 8 squares. For each puff, place filling in the center. Brush the edge of one half with egg wash and fold in half diagonally to form a triangle. Press down the edges with a fork to seal.
Brush & Garnish. Then brush the top of the puff with egg wash and sprinkle with a bit of fresh parmesan, if using. Make 2-3 small slits in the top to allow steam to escape during baking.
Bake at 400 degrees for 15-18 minutes, or until golden.