Char the onion and ginger over an open flame or under a high broiler until the skins are blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until their aroma is released.
Add the beef bone broth, charred onion, charred ginger, and fish sauce to the pot.
Simmer the broth gently for 20 minutes to infuse the flavors, then strain out the solids and season with sea salt and black pepper.
While the broth simmers, cook the rice noodles according to the package directions until tender, then drain and set aside.
Slice the beef eye of round into paper-thin strips against the grain for maximum tenderness.
Divide the cooked noodles into a large bowl, top with the raw beef slices, and pour the boiling hot broth directly over the meat to cook it instantly.
Garnish the bowl with fresh bean sprouts, cilantro, basil, and a squeeze of fresh lime before serving.