Prep the beef. If using a large piece of bone-in chuck, cut the bone away and set aside. Cut the beef into 2" cubes and season with salt and pepper.

Sear the beef. Heat the oil in a Dutch oven over medium-high heat. Add the beef to the pot in a single layer (you may need to do this in batches). Sear until browned on all sides, then remove from the pot using a slotted spoon and transfer to a plate.

Sauté the aromatics. Add the diced onion, garlic, and ginger to the pot and cook for 3-4 minutes, stirring occasionally, until browned and fragrant. Add the gochujang paste and cook for another 2-3 minutes.

Add the beef and broth. Add the beef broth to the pot and stir well to combine. Bring to a gentle simmer and add the seared beef and beef bones. Cover, reduce the heat to medium-low, and simmer for 2 hours or so, until the beef is tender.

Season and finish. Remove the beef bones from the soup and discard. Gently shred the beef, add the rice wine vinegar, fish sauce, and sesame oil to the soup. Season to taste with salt and pepper. You can simmer the noodles directly in the soup or cook separately. Ladle the soup and noodles into bowls and top with chopped kimchi, soft boiled eggs, and green onions.