Char the sliced yellow onion and ginger over a gas flame or under a broiler for 3-5 minutes until slightly blackened and aromatic.
In a medium pot, combine the beef bone broth, charred onion, ginger, star anise, cinnamon stick, sea salt, and black pepper.
Bring the broth to a boil, then reduce the heat to low and simmer for 15-20 minutes to allow the spices to infuse.
While the broth simmers, cook the rice noodles in a separate pot of boiling water according to package directions, then drain and set aside.
Thinly slice the beef eye of round against the grain into paper-thin pieces; for easier slicing, freeze the beef for 20 minutes beforehand.
Stir the fish sauce into the simmering broth and then strain the broth through a fine-mesh sieve to remove the whole spices and onion pieces.
Divide the cooked rice noodles into a large serving bowl and arrange the raw beef slices in a single layer on top of the noodles.
Pour the boiling hot broth directly over the beef to cook it instantly, then garnish with bean sprouts, cilantro, Thai basil, and a squeeze of lime.