Put the sirloin in the freezer. Twenty minutes of firming is what makes paper-thin slicing possible later.
Set a large dry pot over high heat and lay the onion and ginger in cut side down. Leave them 4 to 5 minutes until blackened in spots. The char is most of the shortcut.
Add the star anise, cinnamon, cloves, and coriander seeds to the pot and toast for 1 minute until fragrant. Pour in the stock, add the fish sauce and sugar, cover, and simmer gently for 30 minutes.
While the broth simmers, cook the rice noodles according to the package, rinse under cold water, and divide between 4 deep bowls.
Slice the chilled beef against the grain as thin as you possibly can. Aim for slices you can almost see through. Drape them raw over the noodles.
Strain the broth, return it to the pot, and bring it to a hard rolling boil. Taste and add fish sauce if it needs salt.
Ladle the boiling broth directly over the raw beef. It will cook to rosy pink in the bowl. Serve immediately with the herbs, sprouts, raw onion, and lime on the side.