Char the halved onion and sliced ginger over an open gas flame or under a high broiler until the edges are blackened and fragrant.
In a large stockpot, lightly toast the star anise and cinnamon stick over medium heat for 1-2 minutes until aromatic.
Add the beef broth, charred onion, and charred ginger to the pot, bringing the mixture to a gentle boil.
Reduce the heat and simmer the broth for 20 minutes, then stir in the fish sauce, sea salt, and black pepper.
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 raw beef eye of round against the grain into translucent strips.
Place the cooked noodles in a large bowl and arrange the raw beef slices on top in a single layer.
Ladle the boiling hot broth directly over the beef to cook it instantly, then garnish with bean sprouts, Thai basil, and a fresh squeeze of lime.