Take a baking sheet and line it with parchment paper. Sprinkle some flour on the surface and roll out the puff pastry sheet. Cut the puff pastry sheet in a square shape of the size you require. Place a chicken breast piece in the centre of each square. Spread some cream cheese and mustard on the chicken breasts. Top the chicken breasts with the onion and mushrooms mixture. In the meanwhile, preheat the oven at 190-degree Celsius. Break an egg in a small bowl. Brush the sides of the puff pastry with water. Wrap the sides of the pastry, seal the edges and brush the top with the beaten egg.