CAUTION: Handle peppers carefully. Use of gloves is recommended. Full disclosure: I don’t use gloves. I tried it once and hated the feel. I wash my hands diligently with soap after each contact. So far, I’ve either been terribly lucky or I’m naturally vigilant about keeping my hands from my eyes. That said, it would be terribly painful to get that stuff in your eyes or on other tender parts. Whatever you do, please be careful!
Remove husks from tomatillos. Rinse tomatillos and Serrano peppers. Place them in a pot and cover with water. Bring to boil, then reduce to a low boil, stirring occasionally. Peppers will soften slightly and tomatillos will become dull and possibly split open. When this point is reached, remove from heat, drain in colander, and set aside to cool.
While tomatillos and peppers are cooking and/or cooling, chop cilantro, onion, and garlic cloves. Once the peppers and tomatillos are warm to the touch, trim pepper stems and cut peppers lengthwise to aid in grinding/blending.
If using a blender, gradually place all ingredients except chicken stock together in blender and gradually blend. Add up to one cup of stock if needed to aid in blending.
If using a molcajete, work from dry to wet. Grind dry spices and herbs. Next mash and grind cilantro, garlic and onion. Scrape this mixture into a medium mixing bowl. Mash and grind peppers then tomatillos a few at a time and as it fills scrape into the mixing bowl with the onion and garlic mixture.
A few visual aides for preparation.
Add the mixture to a stock pot along with 4 to 5 cups chicken stock (adjust down if you used some in the blender). This should cover the mixture completely and leave extra liquid to boil down. I recommend starting with 4 cups stock and adding more as needed.
Bring sauce to a boil, then maintain a low boil, stirring occasionally. Do not cover the pot. The sauce will cook down and thicken slightly. The cook time is about 30 minutes, but the sauce can stay on simmer for about 3 hours so long as it is occasionally stirred and covered to avoid too much liquid evaporation.