Place the halved onion and sliced ginger in a dry pot over medium-high heat, charring them until fragrant and slightly blackened on the edges.
Add the beef broth, star anise, and cinnamon stick to the pot, bring to a boil, then reduce heat and simmer for 15 minutes to develop the aromatics.
While the broth simmers, cook the rice noodles in a separate pot of boiling water according to package directions, then drain and place in a large serving bowl.
Stir the fish sauce, sea salt, and black pepper into the broth, then strain the liquid through a fine-mesh sieve to remove the solids.
Arrange the raw, paper-thinly sliced beef eye of round directly on top of the cooked noodles in the bowl.
Pour the boiling hot broth over the beef and noodles, which will instantly cook the meat until it is tender and pink.
Garnish the bowl with bean sprouts, fresh Thai basil, jalapeno slices, and a generous squeeze of lime before serving immediately.