Char the onion and ginger over an open flame or under a broiler until slightly blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick over medium heat for 1-2 minutes until aromatic.
Add the beef bone broth, charred onion, and charred ginger to the pot and bring to a gentle simmer for 20 minutes.
While the broth simmers, soak the rice noodles in hot water according to package directions until tender, then drain and set aside.
Place the beef eye of round in the freezer for 15 minutes to firm up, then slice against the grain into paper-thin pieces.
Strain the broth through a fine-mesh sieve to remove the solids and return the clear liquid to the pot.
Stir in the fish sauce, sea salt, and black pepper, and bring the broth to a rolling boil.
Divide the noodles between deep bowls, arrange the raw beef slices on top, and pour the boiling broth over the meat to cook it instantly.
Garnish with bean sprouts, cilantro, green onion, and a squeeze of lime before serving.