Blanch bones: Cover bones with water, boil 10 minutes, then drain and rinse to remove impurities.
Char aromatics: Lightly oil onion and ginger. Broil or sear until blistered and fragrant, 3–5 minutes.
Toast spices: Warm star anise, cloves, coriander, peppercorns, and cinnamon in a dry pan until fragrant, about 2 minutes.
Simmer broth: Add bones, water, charred aromatics, toasted spices, fish sauce, sugar, and salt. Simmer gently 75–90 minutes, skimming foam.
Strain and reheat: Strain the broth through a fine mesh. Adjust seasoning and bring back to a rolling boil.
Prep noodles: Soak noodles in hot water until pliable, then boil 30–60 seconds. Drain well.
Slice beef: Chill meat 15 minutes to firm. Slice paper-thin across the grain and keep cold.
Assemble bowls: Divide noodles among bowls. Top with raw beef, onion slices, and scallions.
Ladle broth: Pour boiling broth over beef to cook it just until rosy. Add sprouts, herbs, and jalapeño.
Serve hot: Offer lime juice, hoisin, and sriracha so everyone can season to taste.