Prep the vegetables and herbs: Peel the pumpkin or kabocha squash, remove the seeds, and cut it into bite-sized chunks. Finely chop the green onions and cilantro for the garnish, then set them aside.
Start the broth: Place the bite-sized pork spare ribs and halved onion into a large soup pot. Pour in 1.5 litres of fresh water and bring the pot to a boil over medium-high heat.
Skim and simmer: Skim off any foam and impurities that rise to the surface so the broth stays clear. Lower the heat to a gentle simmer, partially cover the pot with the lid, and cook for about 45 minutes until the ribs are tender.
Add the pumpkin: Remove and discard the onion halves from the pot. Carefully add the pumpkin chunks to the broth and let them simmer for a few minutes.
Season gently: Taste the broth first, then season with a few small crystals of rock sugar and a light pinch of salt and mushroom powder, adjusting to your liking. The slow-simmered ribs and pumpkin already provide natural sweetness, so keep the seasoning minimal.
Finish cooking: Continue to simmer gently for about 10 more minutes, or until the pumpkin is fork-tender while still holding its shape nicely. Turn off the heat.
Garnish and serve: Ladle the hot soup, tender ribs, and pumpkin into a serving bowl. Scatter the chopped green onions and cilantro on top, and serve warm alongside a bowl of steamed jasmine rice.