In a skillet, cook the chopped onion with butter over low heat until translucent.
Add flour and cook for a few minutes on low heat.
Gradually whisk in the milk, continuing to cook on low heat until the mixture thickens and is free of lumps.
Stir in the boiled, shredded chicken, salt, black pepper, white pepper, crushed red pepper or chili flakes, and parsley or cilantro. Remove from heat and let the mixture cool completely.
Cut the puff pastry sheets into squares.
Place 1-2 tablespoons of the filling in the center of each square, then cover with another square. Seal the edges by pressing them together with a fork.
Repeat with the remaining squares and place them on a lined cookie sheet.
Beat the egg with a tablespoon of water. Brush each square with egg wash and optionally sprinkle with sesame seeds.
Bake at 350°F for 18-20 minutes or until golden brown.