Char the halved onion and sliced ginger in a dry skillet over medium-high heat until blackened and fragrant.
In a medium pot, toast the star anise and cinnamon stick for 1-2 minutes until they release their aroma.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot; bring to a boil, then reduce heat and simmer for 15 minutes.
While the broth simmers, cook the rice noodles according to package instructions until tender, then drain and set aside.
Thinly slice the raw beef sirloin against the grain into very thin, translucent pieces.
Strain the broth through a fine-mesh sieve to remove the solids, then return the clear liquid to the pot and season with sea salt and black pepper.
Divide the cooked noodles between bowls and arrange the raw beef slices on top.
Pour the boiling hot broth over the beef to cook it instantly, then garnish with bean sprouts, Thai basil, sliced jalapeno, and a fresh squeeze of lime.