Place pork belly on a plate and pat rind dry with paper towel.
Combine soy sauce, mirin, ginger, garlic and chilli paste in a jug. Reserve half the soy sauce mixture. Pour remaining soy sauce mixture into a large glass or ceramic dish. Place pork belly rind-side up in the dish, keeping the rind dry. Sprinkle rind with half the salt. Place in the fridge for 6 hours, or overnight, to marinate.
Preheat oven to 230°C. Pat rind dry with paper towel. Arrange spring onion in a roasting pan. Place pork, rind-side up, on the spring onion. Sprinkle rind with remaining salt. Pour enough water, and any leftover marinade from the dish, into the pan to just cover the onion (make sure the pork rind remains dry). Roast for 30 mins or until the rind begins to crackle. Reduce temperature to 200°C. Roast for a further 1 hour, topping up the water in the pan as necessary. Increase oven temperature to 240°C and roast for a further 20 mins or until rind is crisp and golden. Transfer pork to a plate and set aside for 10 mins to rest. Slice pork crossways. Discard spring onions.
While pork is resting, heat oil in a wok over high heat. Add baby broccoli and buk choy. Cook, stirring, for 2 mins. Add reserved soy sauce mixture and cook, stirring, for 2 mins or until vegetables are tender.
Divide pork belly among serving plates. Drizzle with pan juices and top with coriander leaves. Serve with broccolini mixture and rice.