Char the halved onion and sliced ginger in a dry pan over medium-high heat until blackened spots appear, about 5 minutes.
Place the charred aromatics into a large pot with the beef bone broth, star anise, and cinnamon stick.
Bring the broth to a boil, then reduce heat and simmer for 20 minutes to infuse the deep, aromatic flavors.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Slice the flank steak against the grain into paper-thin pieces; placing the beef in the freezer for 15 minutes beforehand makes this easier.
Strain the broth through a fine-mesh sieve to remove the solids, then stir in the fish sauce and sea salt.
Place the cooked noodles in a large bowl and top with the raw beef slices and fresh bean sprouts.
Pour the boiling hot broth over the beef to cook it instantly, then garnish with cilantro, basil, scallions, and a squeeze of lime.