Place cumin seeds in a cast-iron skillet over medium heat, and cook 2 minutes or until toasted. Transfer seeds to a spice or coffee grinder, and process until finely ground.
Cut 1/4 inch off stem ends of peppers; discard stem ends. Using the tip of a paring knife, carefully remove and discard membranes and seeds, leaving peppers intact.
Heat skillet over medium heat. Add peppers and tomatillos. Cook 15 minutes, turning frequently (peppers and tomatillos will become speckled with black marks).
Transfer contents of skillet to a food processor, and puree until smooth. Add the onion, and pulse 5 times or until blended. Pour pepper mixture into a medium bowl. Stir in cumin, cilantro, lime juice, and salt.