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