Char the onion and ginger over an open flame or under a broiler until slightly blackened and aromatic.
In a medium pot, combine beef broth, water, charred onion, ginger, star anise, and cinnamon stick.
Simmer the broth gently for 20 minutes to develop the deep, spiced flavors.
While broth simmers, cook rice noodles according to package directions, drain, and set aside.
Season the broth with fish sauce, sea salt, and black pepper, then strain out the solids.
Place cooked noodles in a large bowl and top with the raw, thinly sliced sirloin.
Pour the boiling hot broth over the beef to cook it instantly until tender.
Garnish with bean sprouts, cilantro, Thai basil, and a squeeze of lime before serving.