Char the halved onion and sliced ginger in a dry pot over medium-high heat until fragrant and slightly blackened.
Pour in the beef bone broth, star anise, and cinnamon stick; bring to a boil, then reduce heat and simmer for 15 minutes.
While broth simmers, cook the brown rice noodles according to package directions, then drain and set aside.
Slice the flank steak against the grain into very thin, translucent strips and season lightly with sea salt and black pepper.
Strain the broth to remove solids, return to the pot, and stir in the fish sauce.
Divide the cooked noodles into a large bowl and arrange the raw beef slices on top.
Pour the boiling hot broth over the beef to cook it instantly until tender and pink.
Top with bean sprouts, Thai basil, a squeeze of lime, and sriracha before serving.