Lengua Tacos with Tomatillo Salsa

Lengua (beef tongue) is much loved by Mexicans, who will go out of their way to visit a stand that serves it, but it is alien to most Americans, who will go out of their way to avoid it. Tongue may well be the weirdest thing you will ever have in your kitchen, but it is worth the shock value and the time it takes to cook it—the meat is delicious—rich and tender. The cooking process is lengthy but simple: a long simmer, followed by cooling in the cooking stock, then a thorough trimming. Serv