Soak the ox leg bones and the beef flank, brisket, or round in cold water for 20 minutes. Drain and rinse off any bone chips.
Meanwhile, bring a large pot of about 3 ½ quarts of water to a hard boil. Put the bones and beef carefully into the water. Bring back to a boil and continue to cook on high heat for 10 minutes, uncovered. Drain the pot, throwing away the dirty water but reserving the bones and beef. Rinse the beef and bones in cold water and drain. Clean the pot, rinsing off the fat and scum.
Using the same pot, fill it with 3 ½ quarts of water. Add the bones, beef, whole onion, and whole garlic cloves. Bring the pot to a boil over high heat, covered. Once the broth reaches a boil, about 25 minutes later, reduce the heat to low and continue to simmer the broth for 3 hours.
Remove the beef flank from the soup and let cool. Add more water to the pot as it will have reduced. Bring back to a boil, then simmer for another 3 hours.
Add more water to the pot and bring back to a boil. Simmer again for another 3 hours. Continue to cook the broth, adding water every 3 hours if needed, until the soup has achieved a white, milky color. Once the soup is ready, cool and chill overnight in the refrigerator.
Remove the soup from the refrigerator and skim off the hardened fat on top. Bring back to heat.
Slice the beef across the grain, about ⅛ inch thick. Add back to the soup to reheat.
If serving with noodles, cook the somen according to the directions on the package. Drain.
To serve, portion the noodles among 6 bowls. Ladle the soup with the beef per bowl. Serve with green onions, salt, pepper, kimchi, and rice. Allow the guests to season their own soup according to their taste.