Char the onion and ginger over an open flame or under a broiler until blackened and aromatic.
In a large pot, combine beef broth, charred onion, ginger, star anise, and cinnamon stick; simmer for 20 minutes.
Prepare rice noodles according to package directions, then drain and set aside.
Strain the broth to remove solids and return the liquid to the pot, seasoning with fish sauce, salt, and pepper.
Place cooked noodles in a bowl and top with raw, thinly sliced beef eye of round.
Pour the boiling hot broth over the beef to cook it instantly to a tender medium-rare.
Garnish with bean sprouts, Thai basil, jalapeno slices, and a squeeze of fresh lime.