Char the onion and ginger over an open flame or under a broiler until blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick for 1-2 minutes until aromatic.
Add the beef bone broth, charred onion, charred ginger, sea salt, pepper, and fish sauce to the pot.
Simmer the broth gently for at least 20 minutes to develop deep flavors, then strain out the solids.
While broth simmers, cook rice noodles according to package directions, drain, and set aside.
Place the cooked noodles in a large bowl and arrange the raw, thinly sliced beef sirloin on top.
Pour the boiling hot broth over the beef to cook it instantly to a tender medium-rare.
Garnish with bean sprouts, Thai basil, lime wedges, and jalapeño slices before serving.