Fry potatoes in 3 minutes to get a slight crispy layer on top. Set aside
In a stockpot, stew the beef shank (with bone and marrow) with salt, onion, peppercorns, and garlic for at least one and a half hours.
Remove the meat from the marrow after ten minutes or it will dissolve into the broth.
Remove the scum that rises above the soup. Add more hot water if necessary.
When beef is fork-tender, add the potato and let it cook for another 10 minutes.
Add the corn and let it cook for the last 5 minutes, then add cabbage and Chinese cabbage and let it stand for a few minutes.
Season with fish sauce according to taste.
Remove from fire and serve hot in a bowl with the broth mixture consisting of the fish sauce and add some lemon juice to taste.
You can eat it with some rice for a more complete meal