Place the onion and ginger over an open flame or under a broiler until charred and fragrant.
In a large pot, combine the beef bone broth, charred onion, charred ginger, star anise, and cinnamon stick.
Bring the broth to a boil, then reduce heat and simmer for 20 minutes to infuse the flavors.
While the broth simmers, cook the rice stick noodles according to package directions, then drain and set aside.
Slice the beef eye of round against the grain into paper-thin slices; freezing the beef for 15 minutes beforehand makes this easier.
Strain the broth to remove the solids and return the clear liquid to the pot, seasoning with fish sauce, sea salt, and black pepper.
Divide the cooked noodles into a large bowl and layer the raw beef slices on top.
Pour the boiling hot broth over the beef and noodles, which will cook the thin meat instantly.
Garnish with bean sprouts, Thai basil, jalapeño slices, and a squeeze of fresh lime before serving.