Bring a large pot of water to a boil. Add the oxtail and blanch for 5 minutes.
Drain the oxtail and rinse thoroughly under clean water to remove foam and loose proteins.
Transfer the oxtail to the slow cooker. Add onion, ginger, garlic, star anise, cinnamon, peppercorns, soy sauce, sugar, optional Shaoxing wine, and 8 cups of water.
Cover and cook on LOW for 8–9 hours, or until the oxtail is very tender. Begin checking the largest pieces after about 8 hours.
Carefully remove the cooked oxtail and place it on a plate.
Strain the broth through a fine-mesh strainer. Discard the cooked aromatics and whole spices, then skim excess fat from the surface.
Taste the broth and add salt or a little extra soy sauce only if needed.
Cook the noodles separately according to package directions. Drain well.
Blanch the bok choy for 1–2 minutes until bright green and just tender.
Divide the noodles among six serving bowls. Add bok choy and oxtail to each bowl.
Ladle the hot oxtail broth over the noodles and meat.
Garnish with sliced scallions and optional chili crisp, cilantro, rice vinegar, lime, or fresh chili. Serve immediately.
