Char the onion halves and ginger over an open flame or under a broiler until blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick for 2 minutes before adding the beef bone broth.
Add the charred onion, ginger, and fish sauce to the broth; simmer for 20 minutes to develop deep flavors.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Thinly slice the raw beef eye of round against the grain into paper-thin pieces.
Strain the broth to remove solids and return the clear liquid to a boil.
Divide noodles into a bowl, top with raw beef slices, and pour the boiling broth over the meat to cook it instantly.
Season with sea salt and black pepper, then garnish with fresh bean sprouts, Thai basil, and a squeeze of lime before serving.