Slice the beef eye of round against the grain into paper-thin slices and set aside in the refrigerator.
Char the halved onion and sliced ginger over an open flame or under a broiler until slightly blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until aromatic.
Pour the beef broth into the pot with the toasted spices, charred onion, ginger, and fish sauce.
Bring the broth to a boil, then reduce heat and simmer for at least 20 minutes to develop the flavors.
While the broth simmers, cook the rice sticks according to package directions, then drain and divide into serving bowls.
Strain the broth to remove the solids and season with sea salt and black pepper.
Arrange the raw beef slices over the noodles in the bowls.
Pour the boiling hot broth directly over the beef to cook it instantly.
Garnish with bean sprouts, Thai basil, sliced chili, and a squeeze of lime before serving.