From the Freakonomics Podcast,
You cook it [sous-vide style] to perfect medium rare, then you dunk it in liquid nitrogen, which freezes the outside. Then we deep-fry it. We pop it in a deep fryer. Or we use a torch on it, a blowtorch. And either one will give you this incredible crusty outside, but because you put it in liquid nitrogen that prevents it from over cooking, so you get the perfect medium-rare hamburger.