Rinse the oxtail well. Place in a pot. Cover with water. Boil for 10 minutes, drain, discard the water and rinse the oxtail again to remove impurities.
Place the cleaned oxtail in the slow cooker. Add the garlic, scallions, salt and peppercorns. Set the cooker to low and cook the oxtail for eight hours.
Scoop out the oxtail and transfer to a container.
Strain the bone broth into another container.
Chill the oxtail and broth in the fridge overnight or up to a day.
Scrape off the layer of fat that forms on the surface of the broth and discard.
Pour the broth into a pot. Add the oxtail and radish. Simmer until the oxtail meat is tender and the radish is soft.
While the oxtail and radish simmer, make the chili sauce by stirring together all the ingredients.
When the oxtail and radish are done, add the scallions.
Stir in the prepared chili sauce, a tablespoon at a time, until you get the flavor balance that you like.
Ladle the soup into bowls, serve with the remaining chili sauce on the side, and enjoy!