Place the halved onion and ginger cut-side down in a dry skillet over medium-high heat until charred and blackened.
In a medium pot, toast the star anise and cinnamon stick for 1-2 minutes until highly aromatic.
Add the beef bone broth, charred onion, charred ginger, fish sauce, sea salt, and black pepper to the pot; bring to a boil then reduce to a 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.
Place the beef eye of round in the freezer for 10 minutes to firm up, then slice it against the grain into paper-thin strips.
Divide the cooked noodles into a large bowl and arrange the raw beef slices on top of the noodles.
Strain the boiling broth to remove aromatics and immediately pour the hot liquid over the beef to cook it instantly.
Top the bowl with bean sprouts, fresh basil, cilantro, sliced jalapeño, and a squeeze of lime juice before serving.