Thinly slice the beef sirloin against the grain; for easier slicing, freeze the meat for 20 minutes beforehand.
Char the halved onion and ginger over an open flame or under a broiler until slightly blackened and aromatic.
In a large pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until fragrant.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot; simmer gently for 20 minutes to infuse flavors.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Strain the broth 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, thinly sliced beef.
Pour the boiling hot broth over the beef to cook it instantly, then garnish with bean sprouts, Thai basil, jalapeno, and a squeeze of lime.