Place the halved onion and ginger cut side up 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 the aroma is released.
Add the beef bone broth, charred onion, and charred ginger 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.
Slice the beef eye of round against the grain into paper-thin slices; freezing the meat for 15 minutes beforehand makes this easier.
Strain the broth to remove the solids, return the liquid to the pot, and stir in the fish sauce and black pepper.
Divide the cooked noodles and raw beef slices into a large serving bowl.
Pour the boiling hot broth directly over the beef to cook it instantly, then top with bean sprouts, basil, cilantro, sliced chili, and a squeeze of lime.