Place the sliced onion and ginger in a dry pot over medium-high heat, charring them for 2-3 minutes until fragrant and slightly blackened.
Pour the beef bone broth into the pot and add the star anise, cinnamon stick, sea salt, and black pepper.
Bring the broth to a boil, then reduce heat and simmer for 15 minutes to allow the aromatics to infuse the liquid.
While the broth simmers, cook the rice noodles according to package directions, drain, and set aside in a large serving bowl.
Thinly slice the raw flank steak against the grain into paper-thin strips.
Stir the fish sauce into the broth and remove the whole spices (star anise and cinnamon).
Place the raw beef slices on top of the cooked noodles in the bowl.
Pour the boiling hot broth directly over the beef slices to cook them instantly until they turn opaque.
Top the bowl with bean sprouts, fresh cilantro, and Thai basil, serving with a lime wedge on the side for squeezing.