Meanwhile, place a large pot over high heat. Poke the meat all over with a fork to tenderize it and season with salt and pepper. Sear the meat until charred but still rare, 2 to 3 minutes per side, then transfer to a plate. Add the onion and ginger to the pot; cook about 4 minutes. Add the broth, 3 cups water, the star anise and cinnamon, reduce the heat and simmer about 20 minutes.
Meanwhile, thinly slice the scallions and jalapenos (remove seeds for less heat) and tear the cilantro. Thinly slice the meat against the grain. Drain the noodles.
Add the fish sauce to the broth and boil 5 minutes. Discard the ginger, star anise and cinnamon stick. Remove and slice the onion. Divide the noodles among 4 bowls; top with the broth, beef, scallions, cilantro, bean sprouts, jalapenos and onion.