Place the halved onion and sliced ginger under a broiler for 5 minutes until charred and fragrant.
In a medium pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until aromatic.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot. Bring to a boil, then reduce heat and simmer for 20 minutes.
While the broth simmers, prepare the rice noodles according to package directions, then drain and set aside.
Place the beef eye of round in the freezer for 15 minutes to firm up, then slice it against the grain into paper-thin pieces.
Strain the broth to remove the solids and return the clear liquid to the pot. Season with sea salt and black pepper.
Divide the cooked noodles into a large bowl and top with the raw beef slices.
Pour the boiling hot broth over the beef and noodles; the heat of the broth will cook the thin beef instantly.
Garnish with fresh bean sprouts, Thai basil, and a squeeze of lime before serving.