Place the beef eye of round in the freezer for 15-20 minutes to firm up, then slice it against the grain into paper-thin strips.
Char the onion half and sliced ginger over an open flame or under a broiler until blackened in spots to release their deep oils.
In a medium pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until fragrant.
Pour the beef bone broth into the pot with the toasted spices, charred onion, ginger, and fish sauce. Bring to a boil, then reduce heat and simmer for 20 minutes.
While the broth simmers, cook the rice noodles in a separate pot of boiling water according to package directions, then drain and set aside.
Strain the broth through a fine-mesh sieve to remove the solids and return the clear liquid to the pot, seasoning with sea salt and black pepper.
Divide the cooked noodles into a large bowl and arrange the raw beef slices on top of the noodles.
Pour the boiling hot broth directly over the beef slices; the heat from the broth will cook the thin beef instantly to a tender medium-rare.
Top the bowl with fresh mung bean sprouts, Thai basil leaves, jalapeno slices, and a squeeze of lime before serving.