Char the onion and ginger over an open flame or under a broiler until slightly blackened.
In a large pot, toast the star anise and cinnamon stick for 1 minute until fragrant.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot and simmer for 20 minutes.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Thinly slice the eye round steak against the grain into paper-thin pieces.
Strain the broth to remove the solids and return the liquid to the pot, bringing it to a rolling boil.
Place the cooked noodles and raw beef slices into a large bowl.
Pour the boiling broth over the beef and noodles; the heat from the broth will cook the thin beef slices instantly.
Top with bean sprouts, Thai basil, a squeeze of lime, and sriracha before serving.