Char the onion and ginger over an open flame or under a broiler until fragrant and blackened in spots.
In a large pot, toast the star anise and cinnamon stick for 1 minute until the aroma is released.
Add the beef bone broth, charred onion, charred ginger, and fish sauce to the pot and simmer for 20 minutes.
While the broth simmers, soak the rice noodles in hot water according to package directions until tender, then drain.
Thinly slice the raw flank steak against the grain into very thin ribbons.
Strain the broth to remove the solids and return the clear liquid to the pot, seasoning with salt and pepper.
Place the cooked noodles in a large bowl and arrange the raw beef slices on top.
Pour the boiling hot broth over the beef to cook it instantly, then garnish with bean sprouts, basil, and a squeeze of lime.