Char the onions and ginger under the broiler, turning occasionally, until blackened in spots, about 10 minutes.
Toast the star anise, cinnamon, cloves, and coriander seeds in a dry large stockpot over medium heat until fragrant, about 2 minutes.
Add the beef bones, brisket, charred onion and ginger, and water. Bring to a boil, then reduce to a gentle simmer. Skim off any foam.
Simmer gently for 3 hours, until the brisket is tender. Remove the brisket and set aside. Strain the broth through a fine-mesh strainer and return to the pot.
Stir in the fish sauce and rock sugar. Taste and adjust seasoning. Slice the brisket thinly against the grain.
Cook the rice noodles according to package directions. Divide among bowls. Top with brisket slices and raw sirloin slices. Ladle over steaming broth to cook the beef. Serve with bean sprouts, herbs, and lime wedges.