Place the halved onion and sliced ginger face down in a dry pot over medium-high heat until slightly charred and aromatic.
Add the beef bone broth, star anise, and cinnamon stick to the pot. Bring to a boil, then reduce heat and simmer for 15 minutes.
While the broth simmers, prepare the rice noodles according to package instructions, drain, and set aside.
Slice the raw flank steak against the grain into very thin, translucent strips and season lightly with sea salt and black pepper.
Strain the solids from the broth and stir in the fish sauce.
Place the cooked noodles in a deep bowl and top with the raw steak slices.
Pour the boiling hot broth directly over the steak to cook it instantly in the bowl.
Garnish with bean sprouts, cilantro, Thai basil, and a squeeze of fresh lime juice before serving.