In a medium-heated pot, bring water to a rolling boil. Add pork bones, light soy sauce, salt, onions, coriander root, garlic, and black peppercorns. Once boiling, skim off impurities for a clear broth. After skimming, reduce to low heat; simmer for 90 minutes. Then, reserve 27 fl oz of broth for assembling 4 servings of soup later.