Char the halved onion and ginger over an open gas flame or under a high broiler until 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, ginger, and fish sauce to the pot and bring to a gentle simmer for 20 minutes.
While the broth simmers, soak the rice noodles in hot water 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.
Season the broth with sea salt and black pepper, bringing it to a rolling boil.
Divide the cooked noodles into a large bowl and arrange the raw beef slices on top.
Pour the boiling broth over the beef to cook it instantly, then garnish with bean sprouts, cilantro, basil, and a squeeze of lime.