Char the onion and ginger over an open flame or under a broiler until blackened to release their deep, smoky flavors.
In a large pot, combine the beef broth, charred onion, ginger, star anise, and cinnamon stick; bring to a boil then simmer for 20 minutes.
While the broth simmers, soak the rice noodles in hot water according to package directions until tender, then drain.
Slice the raw flank steak against the grain into paper-thin strips for the most tender texture.
Strain the solids from the broth and stir in the fish sauce, sea salt, and black pepper.
Divide the noodles into a bowl and top with the raw beef slices.
Pour the boiling hot broth directly over the beef to cook it instantly in the bowl.
Garnish with bean sprouts, fresh basil, and a squeeze of lime before serving.