Place the halved onion and sliced ginger under a broiler or over an open flame until slightly charred and fragrant.
In a large pot, combine the beef broth, charred onion, ginger, star anise, and cinnamon stick. Bring to a boil, then reduce heat and simmer for 20 minutes.
While the broth simmers, prepare the rice noodles according to package instructions. Drain and set aside.
Place the beef eye of round in the freezer for 15 minutes to firm up, then slice it against the grain into paper-thin pieces.
Strain the solids from the broth and discard them. Stir in the fish sauce, sea salt, and black pepper into the clear liquid.
Divide the cooked noodles into a large bowl and top with the raw beef slices.
Pour the boiling hot broth over the beef and noodles; the heat from the broth will cook the thin beef instantly.
Garnish with bean sprouts, fresh cilantro, Thai basil, and a squeeze of lime before serving.