Place the flank steak in the freezer for 15 minutes to firm up, then slice it against the grain into paper-thin strips.
In a medium pot, char the sliced onion and ginger over medium-high heat for 2-3 minutes until slightly blackened.
Pour the beef bone broth into the pot and add the star anise, cinnamon stick, coconut aminos, and fish sauce.
Bring the broth to a boil, then reduce heat and simmer for 15 minutes to infuse the aromatics.
While the broth simmers, cook the brown rice noodles in a separate pot of boiling water according to package directions, then drain and set aside.
Place the cooked noodles in a large serving bowl and arrange the raw, thinly sliced beef on top of the noodles.
Strain the boiling hot broth to remove the solids and immediately pour it over the beef; the heat from the broth will cook the thin slices of meat instantly.
Top the bowl with bean sprouts, Thai basil, cilantro, and scallions, serving with a lime wedge on the side to squeeze over before eating.