Place beef in a big pot. Add enough water to cover the meat and bring to a boil over medium heat until scum appears on the surface. Remove all the scum or brown bubbles that will form on top of the soup.
Replace water and bring to a boil again. Add garlic, onion, peppercorns & beef cube and fish sauce. Cover and simmer until beef is tender, approximately 1½ – 2 hours. Test for tenderness. Add more water if necessary, i.e., if it has to be cooked longer. Add salt and pepper to suit your taste. Add corn cobs. When almost done (approx. 10 minutes), add pechay and napa cabbage. Simmer for 3 more minutes. Add green onions. Simmer for another minute.