Place the halved onion and sliced ginger under a broiler for 5 minutes until charred and fragrant.
In a medium pot, toast the star anise and cinnamon stick over medium heat for 2 minutes.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot and bring to a gentle simmer for 15 minutes.
While the broth simmers, cook the rice noodles according to package instructions, then drain and set aside.
Place the flank steak 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 salt and pepper.
Divide the cooked noodles into a large bowl and top with the raw, thinly sliced beef.
Pour the boiling hot broth directly over the beef to cook it instantly.
Garnish with bean sprouts, Thai basil, jalapeño slices, and a fresh squeeze of lime juice.