Char the outside of the Onions and a split Ginger over an open flame or under a hot broiler until blackened in spots (about 8–10 minutes), then rinse and set aside.
Parboil the Beef bones and the Beef brisket in a pot of boiling Water for 8–10 minutes to remove impurities; drain, rinse the bones and brisket and wipe the pot clean.
In a dry skillet, toast the Star anise, Cinnamon stick, Cloves, Black cardamom (thảo quả) and Coriander seeds for 2–3 minutes until fragrant, then wrap them in cheesecloth or place in a spice infuser.
Return the cleaned bones and brisket to the pot with fresh Water; add the charred Onions, charred Ginger and the toasted spice bundle. Bring to a boil, then reduce to a gentle simmer.
Simmer gently for 3–4 hours (longer for deeper flavor), skimming any foam or fat that rises. Add the Rock sugar early so it dissolves and balances the savory notes.
About 30–45 minutes before finishing, season the broth with Fish sauce and Salt to taste. Continue to simmer until the Beef brisket is tender.
Remove the brisket and let it cool slightly, then thinly slice against the grain. Strain the broth through a fine sieve or cheesecloth and keep it piping hot.
Prepare the Bánh phở rice noodles according to package instructions (brief soak or quick blanch) and drain. Very thinly slice the partially frozen Beef eye of round against the grain so the pieces are paper-thin.
To assemble, divide the warmed noodles among bowls, arrange a few slices of raw Beef eye of round and some sliced cooked Beef brisket on top, then ladle the boiling hot strained broth over the meat and noodles so the raw beef cooks in the bowl.
Garnish each bowl with chopped Scallions, roughly chopped Cilantro, sprigs of Thai basil and Sawtooth coriander (ngò gai), and a handful of Bean sprouts.
Serve with lime wedges (Limes), sliced Bird's eye chilies, and condiments like Hoisin sauce and Sriracha sauce so everyone can adjust sweetness, heat and salt to taste.
