In an ungreased skillet, cook tomatillos over medium heat until browned, about 10 minutes, turning frequently. Cool on a wire rack. For a milder flavor, remove veins and seeds from serrano peppers; finely chop peppers.
In a food processor, combine tomatillos, peppers, onion, 2 tablespoons cilantro, garlic and salt; cover and pulse just until mixture is coarsely pureed.
Transfer to a serving bowl; sprinkle with remaining cilantro. Refrigerate until serving.