Char the onion and ginger over an open flame or in a dry pan until fragrant and slightly blackened.
In a large pot, combine the beef bone broth, star anise, cinnamon, charred onion, and ginger; simmer for 20 minutes to develop the flavor.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Thinly slice the flank steak against the grain into very thin, translucent strips.
Strain the broth to remove the solids and return the liquid to the pot; season with fish sauce, sea salt, and black pepper.
Place the cooked noodles in a large serving bowl and arrange the raw beef slices on top in a single layer.
Pour the boiling hot broth directly over the beef slices to cook them instantly until they are just tender.
Garnish the bowl with fresh bean sprouts, basil, cilantro, and a squeeze of fresh lime before serving.