Char onion and ginger over an open flame or under a broiler until blackened.
In a large pot, toast star anise and cinnamon stick for 2 minutes until fragrant.
Add bone broth, charred onion, ginger, and fish sauce; simmer for 20 minutes.
While broth simmers, cook rice noodles according to package directions, then drain.
Thinly slice the raw flank steak against the grain into paper-thin pieces.
Strain the broth to remove solids and return liquid to a boil; season with salt and pepper.
Divide noodles into a large bowl and top with the raw beef slices.
Pour the boiling broth over the beef to cook it instantly, then garnish with sprouts, basil, jalapeño, and lime.