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.
Add the beef bone broth, charred onion, charred ginger, and fish sauce to the pot; bring to a boil, then reduce heat and 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 it against the grain into paper-thin pieces.
Strain the broth to remove the solids and season with sea salt and black pepper.
Divide the cooked noodles into a large bowl and arrange the raw beef slices on top.
Pour the boiling hot broth over the beef to cook it instantly, then garnish with bean sprouts, Thai basil, and a squeeze of lime.