Fill a big pot with 3 / 4 full of water. Bring to a boil and gently drop beef shank pieces. Lower the heat to medium and boil (gentle) for about 10 minutes. Remove any dark scum that rises to the top. Discard liquid and rinse meat well in cold water to remove impurities especially by the bone.
Wash the pot and replace the cleaned meat. Fill with water. Add the onion, salt, peppercorns and bring to a boil on medium-high heat. When it starts to boil, skim off any scum that rises to the top and reduce heat to low and simmer until meat is tender** (see note)
Add corn and cook for 5-8 minutes. Add the bok choy or cabbage and cook for another 3 minutes. Turn off heat and add green onion (if using)
Always serve hot with patis-lemon sauce