Prepare Bones and Meat: Blanch beef bones and brisket in boiling water for 5 minutes. Drain, rinse well, and set aside.
Char Onion and Ginger: Place onion and ginger directly over an open flame or under a broiler until charred and fragrant.
Toast Spices: In a dry pan, toast star anise, cinnamon, cloves, coriander, and fennel seeds until aromatic.
Simmer Broth: In a large pot, add bones, brisket, charred onion, ginger, toasted spices, fish sauce, salt, sugar, and water. Bring to a boil, reduce heat, and simmer for 2 ½–3 hours, skimming foam as needed.
Strain Broth: Remove bones, brisket, onion, ginger, and spices. Strain broth through a fine sieve. Slice cooked brisket for serving.
Cook Noodles: Cook rice noodles according to package instructions, rinse, and divide into bowls.
Assemble: Top noodles with sliced raw sirloin, cooked brisket, and ladle hot broth over. The hot broth will cook the thin raw beef instantly.
Serve with Garnishes: Place fresh herbs, sprouts, lime, and sauces on the table for each person to customize their bowl.