Char the halved onion and ginger over an open flame or under a broiler until blackened and fragrant.
Toast the star anise and cinnamon stick in a large pot over medium heat for 2 minutes until aromatic.
Add the beef bone broth, charred onion, ginger, and sea salt to the pot; bring to a boil then simmer gently for 20 minutes.
While the broth simmers, prepare the rice noodles according to package instructions, then drain and place in a large serving bowl.
Slice the raw flank steak against the grain into paper-thin pieces.
Strain the broth through a fine-mesh sieve to remove solids, return the liquid to the pot, and stir in the fish sauce.
Arrange the raw beef slices over the noodles and pour the boiling hot broth directly over the meat to cook it instantly.
Garnish with bean sprouts, cilantro, sliced green onion, jalapeño, and a squeeze of fresh lime juice.