Place the white onion and ginger slices directly over a gas flame or under a broiler until slightly charred and fragrant.
In a large pot, combine the beef bone broth, charred onion, ginger, star anise, and cinnamon stick.
Bring the broth to a boil, then reduce heat and simmer for 20 minutes to infuse the aromatic flavors.
While the broth simmers, cook the brown rice noodles according to package directions, then drain and set aside.
Slice the beef eye of round against the grain into paper-thin pieces (pro tip: freeze for 15 minutes first for easier slicing).
Strain the broth to remove the solids and return the liquid to the pot; stir in the fish sauce, sea salt, and black pepper.
Divide the cooked noodles into bowls and top with the raw beef slices.
Pour the boiling hot broth over the beef to cook it instantly, then garnish with bean sprouts, cilantro, Thai basil, and lime wedges.