Char the onion and ginger over an open flame or under a broiler until the skins are slightly blackened and fragrant.
In a large pot over medium heat, toast the star anise and cinnamon stick for 2 minutes to release their essential oils.
Add the beef bone broth, charred onion, 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 according to package instructions, then drain and set aside.
Place the raw beef in the freezer for 10 minutes to firm up, then slice it against the grain into paper-thin pieces.
Strain the broth through a fine-mesh sieve to remove the solids and return the clear liquid to the pot, bringing it to a rolling boil.
Divide the cooked noodles into a large bowl and arrange the raw beef slices in a single layer on top of the noodles.
Ladle the boiling broth directly over the beef to cook it instantly; season with sea salt and black pepper.
Top the soup with bean sprouts, Thai basil, cilantro, a squeeze of lime, and sriracha before serving.