Place the beef bones into a stock pot and fill with cold water. Bring to the boil, then tip out the water and wash the bones. Return the bones to the pot, refill with fresh cold water and place onto a medium heat to gently simmer. Simmer, turning down to a low heat for as long as you can (about 3-5 hrs) skimming off any scum, and topping up with more water from time to time. Take out the bones and set aside. You can leave the stock to cool completely and freeze portions of it, or make the beef soup from here.
Wash the spinach and leave to drain. Crack the eggs into a bowl and mix lightly with a fork to break the yolks up a little. Tear the nori into small pieces and set aside. Bring the stock to a gentle boil and add the wild rice. Cook for 20mins until the rice is just cooked, then add your beef mince, stirring to break up any large pieces. Add the spinach, then pour a thin stream of egg into the stock so that ribbons form. Season to taste with salt. Place a marrow bone in the middle of large serving bowls. Ladle the hot soup onto the bones, scatter the nori on top, drizzle with sesame oil, dust with white pepper. Serve immediately with kimchi on the side. Happy times.
This dish is best eaten with a generous helping of Kimchi on the side.