Place the halved onion and sliced ginger under a broiler for 5 minutes until charred and fragrant.
In a large pot, lightly toast the star anise and cinnamon stick over medium heat for 2 minutes.
Add the beef broth, charred onion, ginger, and fish sauce to the pot; bring to a boil, then reduce heat and simmer for 20 minutes.
While the broth simmers, cook the rice stick noodles according to package directions, then drain and set aside.
Place the beef top round in the freezer for 10 minutes to firm up, then slice it against the grain into paper-thin strips.
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 top with the raw beef slices.
Pour the boiling hot broth over the beef to cook it instantly, then top with bean sprouts, Thai basil, jalapeño slices, and a squeeze of lime.