Place the halved onion and sliced ginger in a dry skillet over medium-high heat, charring them until blackened in spots to develop a deep, smoky base.
In a large stockpot, toast the star anise and cinnamon stick for 2 minutes until they release their warm aroma.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot; bring to a boil, then reduce heat and simmer for 20 minutes.
While the broth simmers, cook the rice noodles in a separate pot according to package instructions until tender, then drain and set aside.
Strain the finished broth through a fine-mesh sieve into a clean pot, discarding the solids, and return the liquid to a rolling boil.
Divide the cooked noodles into a large serving bowl and arrange the raw, paper-thin beef slices on top of the noodles.
Carefully pour the boiling hot broth over the beef to cook it instantly, then season the soup with sea salt and black pepper.
Top the bowl with fresh bean sprouts and Thai basil, serving immediately with a lime wedge on the side for a bright finish.