Place the halved onion and sliced ginger face down in a dry pot over medium-high heat until slightly charred and aromatic.
Add star anise and the cinnamon stick to the pot, toasting for 1 minute until their scent is released.
Pour in the beef bone broth and fish sauce, bringing the mixture to a boil before reducing to a simmer for 15 minutes.
While the broth simmers, prepare the rice noodles according to package directions and thinly slice the raw flank steak against the grain.
Add the chopped baby bok choy to the simmering broth for the final 2 minutes until tender-crisp.
Place the cooked noodles in a large bowl, top with the raw steak slices, and pour the boiling hot broth directly over the beef to cook it instantly.
Season with sea salt and black pepper, then garnish the bowl with bean sprouts, fresh cilantro, Thai basil, and a generous squeeze of lime juice.