Char the halved yellow onion and sliced ginger over an open gas flame or under a broiler until blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until aromatic.
Add the beef bone broth, charred onion, charred ginger, and fish sauce to the pot and bring to a boil.
Reduce heat and let the broth simmer for 20 minutes to infuse the flavors, then strain out the solids.
While the broth simmers, cook the rice stick noodles according to package directions, drain, and divide into serving bowls.
Thinly slice the raw flank steak against the grain and arrange the slices on top of the cooked noodles.
Season the beef with sea salt and black pepper, then pour the boiling hot broth directly over the meat to cook it instantly.
Garnish each bowl with fresh bean sprouts, Thai basil leaves, jalapeño slices, and a squeeze of fresh lime.