Start by parboiling the knuckle bones. To do this, get a large pot to a rapid boil, then add the knuckle bones and let boil for ten minutes. When the time is up, pour the water out through a strainer to catch the bones. Rinse the pot and lightly rinse the bones then refill the pot with cool water. If you're using marrow bones instead of knuckle bones, scoop out and discard some of the marrow while it's hot (and soft) so the soup base will be clearer.
Char the onions and ginger by bringing your oven rack to the highest rung and setting your oven to broil. Then place the halved onions and ginger on a baking sheet. Brush with olive oil and place under the broiler. Broil for about 6 minutes each side or until the vegetables begin to char.
Add the charred ginger and onion, cinnamon stick, coriander, fennel seeds, anise, cardamom pod, cloves, salt, fish sauce, sugar, and beef and simmer uncovered for 1 1/2 hours. Every so often come back to the pot and skim any scum off the top of the water using a fine mesh skimmer.
Remove the beef meat and set aside for later use. Continue simmering for another 1 1/2 hours.
Strain broth and return the broth to the pot. Taste broth and adjust seasoning as needed. The most common adjustments are to add more of fish sauce (a teaspoon at a time), a large pinch of salt, and a teaspoon of sugar (or a small nugget of rock sugar if you have it). Keep doing this until the broth tastes perfect.
Slice your flank/london broil/sirloin steak as thin as possible. It may help to freeze the steak for about 15 minutes prior to slicing to make it easier. Get the beef that was a part of your broth and cut or shred it and set aside. Arrange all other ingredients on a platter for the table. It's traditional to have your diners assemble their own bowls.
Follow the directions on your package of noodles to cook them til just done. For some fresh rice noodles, just a quick 5 second blanch in hot water is all that's needed.
Bring your broth back to a boil. Line up your soup bowls next to the stove. Fill each bowl with rice noodles, shredded cooked beef and raw meat slices. As soon as the broth comes back to a boil, ladle into each bowl. the hot broth will cook your raw beef slices. Serve immediately. Guests can garnish their own bowls as they wish.
