In a pot, bring 1 liter of water to a boil.
Add pork bones, halved onion, and quartered tomato or radish to the pot.
Season with fish sauce, salt, and sugar.
Simmer for about 45 minutes to 1 hour, occasionally skimming off any foam that rises to the surface.
After simmering, strain the broth to remove the bones and vegetables. Set aside.