Fill a large pot with water and add the beef and bone marrow if using. Over a medium heat. Bring to the boil. Skimming the scum that floats to the top untill the broth is clear.
Add the stock cube, onion, peppercorns and fish sauce. Lower the heat to a simmer. Cover and cook for about 1 to 1 and a half hours. Or until the the beef is tender. Keep an eye on the the water level. Add more if needed.
Once beef is tender, add the green beans. Cook for 5-7 minutes.
Add the leek and cabbage. Cook for a further 5 minutes until the vegetables are cooked. Season to taste.