Char the onion and ginger over an open flame or under a broiler until the skins are blackened and fragrant.
In a medium pot, toast the star anise and cinnamon stick over medium heat for 1 minute until their oils are released.
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 boiling water according to package instructions, then drain and divide into a large bowl.
Season the broth with sea salt and black pepper, then strain through a fine-mesh sieve to remove the solids.
Arrange the raw, thinly sliced beef sirloin in a single layer over the cooked noodles.
Pour the boiling hot broth directly over the beef slices to cook them instantly until they are just tender.
Garnish the bowl with fresh bean sprouts, cilantro, Thai basil, and a squeeze of lime before serving.