Preheat oven to 180C. Combine soya sauce, sugar, five spice, orange juice and ginger, roll pork in marinade. Heat 1tbsp oil in large frying pan over high heat; fry pork for 5-6mins until brown all over. Reserve marinade for later.
Transfer pork to roasting tray and place in oven for 25mins, pouring in marinade after 15mins. Meanwhile, wipe out frying pan, return it to medium-high heat and toast sesame seeds for 1-2mins until golden; set aside.
Add remaining oil, garlic and pak choi to pan. Cook over medium heat, tossing for 1min, then add 3tbsp water and cook for 4-5mins until water has evaporated. Trickle over sesame oil and pile onto plates; top with sliced pork and sesame seeds. Spoon over any juices from roasting tray.