Char the onion and ginger under a broiler until blackened and softened.
Toast star anise and cinnamon in a large pot over medium heat until aromatic.
Pour in the beef bone broth and add the charred onion, ginger, and fish sauce.
Simmer the broth gently for 20 minutes to develop deep flavors.
Prepare rice noodles by soaking in hot water until tender, then drain and set aside.
Slice the beef against the grain into very thin, translucent strips.
Strain the broth through a fine-mesh sieve and return the liquid to a rolling boil.
Divide noodles into a bowl, layer raw beef on top, and pour the boiling broth over to cook the meat instantly.
Finish with bean sprouts, cilantro, Thai basil, a squeeze of lime, salt, and pepper.