Char the onion and ginger over an open flame or under a broiler until slightly blackened to release deep, smoky flavors.
In a large pot, toast the star anise and cinnamon stick for 1-2 minutes until fragrant.
Add the beef broth, charred onion, and ginger to the pot; simmer for 20 minutes to infuse the aromatics.
While the broth simmers, cook the rice noodles according to package instructions, drain, and set aside.
Strain the broth to remove solids, then stir in the fish sauce, sea salt, and black pepper.
Divide the cooked noodles into a large bowl and layer the raw, thinly sliced beef on top.
Pour the boiling hot broth over the beef to cook it instantly, then garnish with bean sprouts, basil, and a squeeze of lime.