Char the onion and ginger over an open flame or under a broiler until slightly blackened to release deep aromatic flavors.
In a large pot, bring the beef broth to a simmer with star anise, cinnamon, charred onion, ginger, and fish sauce.
Let the broth steep for 20 minutes on low heat, then strain out the solids and season with sea salt and black pepper.
While the broth simmers, cook the rice sticks according to package directions until tender, then drain and set aside.
Arrange the cooked noodles in a large bowl and top with the raw, thinly sliced beef eye of round.
Pour the boiling hot broth directly over the beef to cook it instantly in the bowl.
Garnish with bean sprouts, Thai basil, sliced jalapeno, and a squeeze of fresh lime before serving.