Blanch the pork bones: Bring a pot of water to a boil. Add the pork bones and cook for 3-5 minutes to remove impurities. Drain and rinse the bones under cold water.
Make the broth: In a clean pot, add the blanched bones, 2 liters of water, onion, ginger, and salt. Bring to a boil, then lower the heat and simmer for 1-2 hours to extract all the flavors. Skim off any foam that rises to the top for a clearer broth.
Season: After simmering, remove the bones and vegetables from the broth. Season with fish sauce, sugar, and additional salt to taste. Keep the broth warm on low heat.