Take a large pot and fill it with water
Add the oxtails to the water and parboil them for 30 minutes.
Oil and excess fat will start forming a foam layer on the water surface, removing them.
Add the star anise, orange peel, ginger, and salt to the boiling mixture and cook them at a simmer.
Keep it at a simmer for 1 hour to cook the oxtails till tender.
Add the peanuts to the pot and boil it for another 2 to 3 hours.
To decide if the cooking is complete, check for meat falling off the bones. If this is seen, be sure that the oxtails are cooked to perfection.
Keep skimming the fat from the boiling mixtures periodically. Having fat in the dish will alter the taste.
Add the mustard greens and chili pepper flakes.
Cook for another 5 to 7 minutes until the mustard greens become tender.
Remove the pot from the stove and garnish it with green onions, ginger, and cilantro to serve.
You can serve it hot with rice for the best taste.