Season the spare ribs with salt, white pepper and soy sauce to taste. You may also use pork neck bones or other meaty soup bones for this recipe. Bring the water, the bouillon cube and the ribs to a boil over high heat. Reduce heat and simmer for 30 minutes, skimming the top as the meat renders. Taste the broth and add soy sauce if the soup is bland.
Wash the watercress thoroughly and add to the soup, stirring occasionally. Simmer for 15 minutes and serve.