Place the halved onion and sliced ginger in a dry pan over medium-high heat, charring them until fragrant and slightly blackened on the edges.
In a medium pot, combine the beef broth, charred onion, ginger, star anise, and cinnamon stick; bring to a boil and then simmer for 20 minutes.
While the broth infuses, cook the rice noodles according to package instructions until tender, then drain and set aside.
Place the flank steak in the freezer for 10 minutes to firm up, then slice it against the grain into paper-thin strips.
Strain the broth through a fine-mesh sieve to remove the solids, return the clear liquid to the pot, and stir in the fish sauce, sea salt, and black pepper.
Divide the cooked noodles into 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 to cook it instantly, then top with bean sprouts, Thai basil, and a squeeze of lime.