Cook rice noodles according to packet instructions. Set aside and keep warm
Place char siu and five spice powder in a small jug and mix to combine. Spread 2 tablespoons of mixture onto pork. Add soy sauce to remaining mixture, stir to combine.
Heat oil in large deep frying pan over medium heat. Add pork, and stir-fry for 6-8 minutes, turning until browned all over. Add mushrooms, cook stirring for 2 minutes, until golden. Add remaining marinade and water to pan. and bring to a gentle simmer, about 5 minutes. Remove pork and set aside to rest.
Add gai lan to frying pan, cover and cook 2-3 minutes or until tender. Thickly slice pork and serve with mushrooms and gai larn and sprinkle with green onion and black sesame. Serve with rice noodles.