Discard husks and rinse tomatillos. In a 1- to 1 1/2-quart pan, combine tomatillos and 2 cups water. Bring to a boil over high heat. Cover and simmer until tomatillos are soft when pressed, 7 to 10 minutes. With a slotted spoon, transfer tomatillos to a blender or food processor; reserve cooking water.
Rinse, stem, and, if desired for less heat, seed chili. Coarsely chop chili and add to tomatillos along with 2 tablespoons cooking water; whirl until smooth. Pour into a bowl.
When cool, add onions, cilantro, garlic, and salt and pepper to taste. For thinner salsa, add more of the cooking water.
Nutritional analysis per tablespoon.