Begin by filling a large pot with enough water to completely submerge the oxtails. Bring the water to a vigorous boil over high heat, then add the oxtails and let them parboil for 30 minutes. After 30 minutes, drain the oxtails and rinse them under cold water until no scum remains.

Return the cleaned oxtails to the pot, covering with fresh water and adding dried orange peel, star anise, fresh ginger, and a generous pinch of salt. Bring to a rapid boil, then reduce to a gentle simmer, covering the pot. Let it simmer for about 1 hour.

Stir in the raw peanuts and continue to let the soup simmer gently for an additional 2 to 3 hours. Skim off any foam that rises to the top.

For a cleaner tasting soup, allow the pot to cool down before refrigerating overnight to solidify the fat at the top, making it easy to skim off.

When ready to serve, reheat the soup on medium heat until heated through. Add chili pepper flakes and mustard greens, allowing them to wilt, about 5 minutes.

Ladle the soup into bowls, including oxtail, peanuts, and greens. Finish with fresh cilantro, green onions, and optionally, grated ginger.