Broth:Boil bones for 10-15 minutes to remove impurities, then rinse thoroughly.
Char Aromatics: Char onion and ginger over an open flame or under a broiler until blackened, then peel.
Simmer:Simmer bones with charred aromatics and toasted spices in a large pot for at least 6-8 hours (skimming foam regularly).
Season:Season with fish sauce, rock sugar, and salt to taste.
Serve: Place blanched noodles in a bowl, top with raw beef slices, and pour boiling broth over to cook the meat instantly.