Char the onion and ginger over an open flame or under a broiler until slightly blackened to release deep aromatics.
In a large pot, toast the star anise and cinnamon stick over medium heat for 2 minutes until fragrant.
Add the beef bone broth, charred onion, ginger, and sea salt to the pot; bring to a boil, then reduce heat and simmer for 20 minutes.
While the broth simmers, cook the rice noodles according to package directions, drain, and set aside.
Strain the broth to remove the solids and stir in the fish sauce for a savory depth.
Divide the cooked noodles into a large bowl and top with the raw, thinly sliced beef.
Pour the boiling hot broth over the beef to cook it instantly to a tender medium-rare.
Garnish with bean sprouts, Thai basil, a squeeze of lime, and sriracha before serving.