Prepare the dry rice noodles according to the package instructions by soaking in hot water until tender, then drain and set aside.
In a medium pot over medium-high heat, char the sliced red onion and ginger for 2-3 minutes until they are slightly blackened and fragrant.
Pour the beef broth into the pot and add the star anise, cinnamon stick, tamari, sea salt, and black pepper.
Bring the broth to a boil, then reduce the heat to low and simmer for 15 minutes to allow the spices to fully infuse the liquid.
While the broth is simmering, thinly slice the flank steak against the grain into very thin, translucent ribbons.
Remove the star anise and cinnamon stick from the broth and stir in the toasted sesame oil.
Place the prepared noodles into a large serving bowl and arrange the raw beef slices on top of the noodles.
Ladle the boiling hot broth directly over the beef slices to cook them instantly, then garnish the bowl with bean sprouts, fresh cilantro, and Thai basil.