Char the onion and ginger over an open flame or under a broiler until slightly blackened and aromatic.
In a large pot, toast the star anise and cinnamon stick for 1-2 minutes, then add the bone broth, charred onion, ginger, and fish sauce.
Bring the broth to a boil, then reduce heat and simmer for 20 minutes to develop the deep, savory flavors.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Thinly slice the raw beef against the grain; for easier slicing, freeze the meat for 15 minutes prior to cutting.
Strain the broth to remove the solids and return the liquid to the pot, seasoning with sea salt and black pepper.
Divide the cooked noodles into bowls, top with the raw beef slices, and pour the boiling hot broth over the meat to cook it instantly.
Garnish with bean sprouts, fresh Thai basil, and a squeeze of lime before serving.