Char the onion and ginger over an open flame or in a dry pan until fragrant and slightly blackened.
In a large pot, toast the star anise and cinnamon stick for 1-2 minutes until the aroma fills the kitchen.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot and simmer for 20 minutes.
While the broth simmers, prepare the rice noodles according to package directions and set aside.
Thinly slice the raw flank steak against the grain into paper-thin strips.
Strain the broth to remove the solids and return the clear liquid to the pot, bringing it to a rolling boil.
Place the cooked noodles in a large bowl and top with the raw beef slices.
Pour the boiling broth over the beef to cook it instantly, then garnish with bean sprouts, basil, cilantro, jalapeño, and a squeeze of lime.