Place the halved onion and ginger cut-side down in a dry pan over medium-high heat until deeply charred and fragrant.
In a large pot, lightly toast the star anise and cinnamon stick for 1 minute before adding the beef bone broth, charred onion, and ginger.
Bring the broth to a boil, then reduce heat to low and simmer for 20 minutes to infuse the aromatic flavors.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Thinly slice the beef flank steak against the grain into paper-thin pieces while the meat is cold.
Strain the broth to remove the solids and stir in the fish sauce, sea salt, and black pepper.
Divide the cooked noodles into bowls, top with the raw beef slices and bean sprouts, then pour the boiling hot broth over the beef to cook it instantly.
Garnish each bowl with fresh cilantro, Thai basil, jalapeno slices, and a generous squeeze of lime.