Place the halved onion and sliced ginger in a dry skillet over medium-high heat, charring until fragrant and slightly blackened on the edges.
In a large pot, combine the beef bone broth, star anise, cinnamon stick, charred onion, ginger, and fish sauce.
Bring the broth to a boil, then reduce heat and simmer uncovered for 20 minutes to allow the spices to infuse the liquid.
While the broth simmers, cook the rice noodles according to package directions until tender, then drain and set aside.
Strain the finished broth through a fine-mesh sieve to remove the solids and return the clear liquid to the pot.
Season the broth with sea salt and black pepper, keeping it at a rolling boil.
Divide the cooked noodles into a large bowl and arrange the raw, paper-thin slices of flank steak over the top.
Pour the boiling broth directly over the beef to cook it instantly, then garnish with bean sprouts, basil, cilantro, and a squeeze of lime.