Char the onion and ginger over an open flame or under a broiler until blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick for 1-2 minutes until aromatic.
Add the beef broth, charred onion, ginger, and fish sauce to the pot; simmer for 20 minutes to develop deep flavors.
While the broth simmers, cook the rice noodles according to package directions, then drain and set aside.
Strain the broth to remove solids and return the clear liquid to the pot, seasoning with sea salt and black pepper.
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 until tender and pink.
Garnish with bean sprouts, basil, cilantro, jalapeño slices, and a fresh squeeze of lime.