Season oxtails with salt and pepper. Heat a large pot with 1 tbsp oil. Sear the oxtails on all sides. Remove from the pan and add the onion, celery, and carrot. Cook until the onions start to get soft and brown. Add sherry, soy sauce, water and spices. Add the oxtails back to the pot. Bring to a boil and reduce to a low simmer. Cook for 3 hours or until the oxtails are tender. Make the wonton wrappers.
Remove the oxtails from the broth. Allow to cool slightly. Shred the meat, removing the bone, fat, and cartilage. Strain the broth and return to the pot.
After the wontons have been made, bring the broth to a boil. Add the wontons, cook for one minute. Add the bok choy and carrots. Cook until the wonton and vegetables are tender. Season to taste. Garnish with green onions and serve.