Char the halved onion and ginger over an open flame or under a broiler until slightly blackened to release deep, smoky flavors.
In a large pot, toast the star anise and cinnamon stick for 1-2 minutes until fragrant, then add the beef broth, charred onion, and ginger.
Simmer the broth gently for 20 minutes, then stir in the fish sauce and sea salt before straining the solids out to leave a clear, aromatic liquid.
While the broth simmers, cook the rice noodles according to package directions, drain, and divide into serving bowls.
Thinly slice the raw beef sirloin against the grain into paper-thin pieces using a very sharp knife.
Place the raw beef slices over the noodles and pour the boiling hot broth directly over the meat to cook it instantly.
Top the soup with bean sprouts, fresh cilantro, Thai basil, black pepper, and a generous squeeze of lime before serving.