Char the onion and ginger over an open flame or under a broiler until fragrant and slightly blackened.
In a large pot, toast the star anise and cinnamon stick for 1-2 minutes until aromatic.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot and simmer for 20 minutes.
While the broth simmers, cook the rice noodles according to package directions and drain.
Thinly slice the raw flank steak against the grain into very thin ribbons.
Strain the broth to remove solids and return the clear liquid to the pot, seasoning with sea salt and black pepper.
Place the cooked noodles in a bowl, top with raw beef slices, and pour the boiling broth over the beef to cook it instantly.
Garnish with bean sprouts, Thai basil, lime wedges, and sriracha.