Char the halved yellow onion and sliced ginger under a broiler or over a gas flame until the edges are slightly blackened.
Place the star anise and cinnamon stick in a dry pot over medium heat for 2 minutes until they become highly fragrant.
Pour the beef bone broth into the pot and add the charred onion, ginger, fish sauce, sea salt, and black pepper.
Bring the broth to a boil, then reduce heat and simmer gently for 20 minutes to infuse the flavors.
While the broth simmers, cook the rice stick noodles according to the package instructions, then drain and set aside.
Slice the raw flank steak against the grain into very thin, translucent strips.
Strain the solids from the broth and return the liquid to a rolling boil.
Divide the cooked noodles into a large bowl, layer the raw beef strips on top, and pour the boiling broth over the meat to cook it instantly.
Garnish with bean sprouts, Thai basil, jalapeño slices, and a fresh squeeze of lime before serving.