Char the halved onion and ginger over an open flame or under a broiler until slightly blackened and fragrant.
In a large pot, combine the beef bone broth, charred onion, ginger, star anise, and cinnamon stick.
Bring the liquid to a boil, then reduce heat and simmer for 20 minutes to allow the aromatics to infuse the broth.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Strain the broth into a clean pot, discarding the solids, and stir in the fish sauce, sea salt, and black pepper.
Arrange the cooked noodles in a large deep bowl and top with the raw, very thinly sliced beef eye of round.
Pour the boiling hot broth directly over the beef to cook it instantly in the bowl.
Garnish the soup with bean sprouts, cilantro, Thai basil, jalapeno, and a squeeze of fresh lime juice before serving.