Char the onion and ginger over an open flame or under a broiler until blackened and fragrant.
In a large pot, combine the beef bone broth, charred onion, charred ginger, star anise, and cinnamon stick.
Bring the broth to a boil, then reduce heat and simmer for 20 minutes to infuse the flavors.
While the broth simmers, cook the rice stick noodles according to package directions, drain, and set aside.
Thinly slice the beef eye of round against the grain into paper-thin pieces.
Strain the broth to remove the solids and return the liquid to the pot; season with fish sauce, sea salt, and black pepper.
Divide the cooked noodles into bowls and arrange the raw beef slices on top in a single layer.
Pour the boiling hot broth over the beef to cook it instantly, then garnish with bean sprouts, Thai basil, cilantro, and a squeeze of lime.