Place the halved onion and sliced ginger under a broiler for 5 minutes until charred and fragrant.
In a medium pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until aromatic.
Add the beef bone broth, charred onion, charred ginger, and fish sauce to the pot; bring to a boil, then reduce heat and simmer for 20 minutes.
While the broth simmers, prepare the rice noodles according to package directions, then drain and set aside.
Slice the beef flank steak against the grain into paper-thin strips; seasoning lightly with sea salt and black pepper.
Strain the broth to remove the solids and return the clear liquid to the pot, keeping it at a rolling boil.
Divide the cooked noodles into a large bowl and arrange the raw beef slices on top.
Pour the boiling broth directly over the beef to cook it instantly in the bowl.
Top with bean sprouts, Thai basil, a squeeze of lime, and a drizzle of coconut aminos before serving.