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, ginger, star anise, and cinnamon stick.
Bring to a boil, then reduce heat and simmer for 20 minutes to infuse the flavors.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Strain the broth to remove the solids and return the liquid to the pot; stir in the fish sauce, salt, and pepper.
Divide the cooked noodles into a large bowl and top with the raw, thinly sliced beef eye of round.
Pour the boiling hot broth over the beef to cook it instantly until it reaches a tender, pink finish.
Garnish with bean sprouts, Thai basil, sliced scallions, and a squeeze of fresh lime juice.