Char the halved yellow onion and the piece of ginger over an open gas flame or under a broiler until the skins are blackened and fragrant.
In a large stockpot over medium heat, toast the star anise and cinnamon stick for 2 minutes until their aroma is released.
Add the beef bone broth, charred onion, charred ginger, and fish sauce to the pot and bring to a gentle simmer for 20 minutes.
While the broth simmers, cook the rice noodles in a separate pot of boiling water according to package instructions, then drain and set aside.
Strain the broth through a fine-mesh sieve to remove the solids and return the clear liquid to the pot, seasoning with sea salt and black pepper.
Divide the cooked noodles into a large serving bowl and arrange the raw, thinly sliced beef eye of round on top of the noodles.
Pour the boiling hot broth directly over the raw beef slices, which will cook the meat instantly to a perfect, tender medium-rare.
Top the soup with fresh bean sprouts, Thai basil leaves, sliced scallions, and a generous squeeze of lime juice before serving.