Place halved onion and ginger cut-side down in a dry pan over medium-high heat until charred and aromatic.
In a large pot, combine beef bone broth, charred onion, ginger, star anise, and cinnamon stick; simmer for 20 minutes.
While broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Slice the raw beef against the grain into paper-thin pieces to ensure they cook instantly in the hot broth.
Strain the broth into a clean pot, discarding solids, and stir in the fish sauce, sea salt, and black pepper.
Divide noodles between bowls, top with raw beef slices, and pour the boiling broth over the meat to cook it.
Garnish with mung bean sprouts, cilantro, Thai basil, and scallions, serving with a fresh lime wedge on the side.