Place the halved onion and ginger over an open flame or under a broiler until charred and fragrant.
In a large pot, lightly toast the star anise and cinnamon stick for 1-2 minutes until aromatic.
Pour in the beef bone broth and add the charred onion, ginger, sea salt, and black pepper.
Bring the broth to a boil, then reduce the heat and simmer for 20 minutes to develop the deep flavors.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Stir the fish sauce into the broth and remove the large aromatics including the onion, ginger, and whole spices.
Place the cooked noodles and the raw, very thinly sliced beef into a deep serving bowl.
Pour the boiling hot broth over the beef to cook it instantly to a tender medium-rare consistency.
Top the soup with bean sprouts, fresh Thai basil, jalapeño slices, and a squeeze of fresh lime juice before serving.