Char the onion and ginger over an open flame or under a broiler until slightly blackened to release aromatics.
In a large pot, combine the beef bone broth, star anise, cinnamon stick, charred onion, and ginger.
Bring the broth to a boil, then reduce heat and simmer for 20 minutes to infuse the liquid with spices.
While the broth simmers, cook the rice noodles according to package directions, drain, and set aside in a serving bowl.
Thinly slice the raw beef eye of round against the grain into paper-thin pieces for quick cooking.
Strain the broth to remove the solids and return the liquid to the pot, seasoning with fish sauce, salt, and pepper.
Arrange the raw beef slices over the cooked noodles in the serving bowl.
Pour the boiling hot broth directly over the beef slices to cook them instantly in the bowl.
Top the soup with bean sprouts, Thai basil, a squeeze of lime, and sriracha before serving.