Place the flank steak in the freezer for 15 minutes to firm up, then slice it against the grain into paper-thin strips.
In a dry pot over medium-high heat, char the onion and ginger until fragrant and slightly blackened.
Add the beef bone broth, star anise, and cinnamon stick to the pot and bring to a gentle simmer for 15 minutes.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Stir the fish sauce, sea salt, and black pepper into the broth and remove the whole spices and onion/ginger pieces.
Place the cooked noodles in a large bowl and arrange the raw beef slices on top.
Pour the boiling hot broth over the beef and noodles, which will cook the thin steak instantly.
Top with fresh bean sprouts, Thai basil, jalapeño slices, and a squeeze of lime before serving.