Char the onion and ginger over a gas flame or under the broiler until the skins are blackened and fragrant.
In a large pot, combine the beef bone broth, charred onion, ginger, star anise, and cinnamon stick; simmer for 20 minutes.
Cook the brown rice noodles in a separate pot of boiling water until tender, then drain and rinse.
Strain the broth into a clean pot, discarding the solids, and stir in the fish sauce, sea salt, and black pepper.
Divide the noodles into a large serving bowl and arrange the raw, thinly sliced flank steak on top.
Ladle the boiling hot broth directly over the steak to cook it through while maintaining a succulent texture.
Top the soup with bean sprouts, Thai basil, jalapeno slices, and a fresh squeeze of lime before serving.