How to Make It
Place beef shanks in a large stockpot, and cover with cold water. Bring to a boil over high; reduce heat to medium-low, and simmer 10 minutes. Skim and discard any scum and fat that come to surface. Remove beef shanks, and set aside. Discard cooking liquid. Clean pot. (This step guarantees a clean and clear broth.)
Heat oil in pot over medium-high. Add onion, garlic, and ginger; cook, stirring occasionally, until onion is translucent and mixture is aromatic, about 3 minutes.
Return beef shanks to pot; add stock. Bring to a boil over high; reduce heat to medium-low to maintain a simmer. Skim and discard any scum that surfaces. Add fish sauce and black pepper; simmer until beef is tender, 2 hours and 30 minutes to 3 hours.
Add chile slices, if using, and simmer 3 minutes. Add corn; simmer 6 minutes. Add green beans; simmer 3 minutes. Add cabbage, and simmer until crisp-tender, about 3 minutes. Add fish sauce and black pepper to taste.