Husk and wash the tomatillos very well. Cut into large pieces. Place in enough water to cover, and bring to boil. Reduce heat and add chopped onion. Boil for 10 minutes or until onion is soft. Remove from heat. Drain off most of water and reserve.
Once cooled, place the tomatillos and onion in blender. Add some reserved water if needed. Blend until fairly smooth. Add the chile peppers and cilantro and blend again. Season to taste with lime juice and salt.
This is a really good salsa for enchiladas. If you want a slightly chunkier salsa, discard all of the boiling water before blending.