Place the onion and ginger in a dry skillet over medium-high heat, charring them until they are fragrant and slightly blackened on the edges.
In a large pot, combine the beef bone broth, charred onion, ginger, star anise, and cinnamon stick, then bring to a gentle simmer for 20 minutes.
While the broth develops flavor, prepare the rice stick noodles by soaking them in hot water according to the package instructions until they are silky and tender.
Strain the broth through a fine-mesh sieve to remove the solids, then return the clear liquid to the pot and stir in the fish sauce, sea salt, and black pepper.
Divide the cooked noodles between bowls and layer the raw, paper-thin slices of beef eye of round directly on top of the noodles.
Ladle the boiling hot broth over the beef to cook it instantly, then garnish with bean sprouts, Thai basil, sliced green onion, and a fresh lime wedge.