1. In a 2- to 3-quart pan over high heat, bring 4 cups water to a boil. Husk and rinse tomatillos, then add to boiling water. Cook until barely soft when pressed, about 5 minutes.
2. Drain tomatillos and immerse in ice water. When cool, in about 3 minutes, purée in a blender or food processor. Pour purée into a bowl.
3. Add red onion, chili, lime juice, and cilantro and mix. Season to taste with salt and pepper.
Nutritional analysis per tablespoon.