Salsa Verde
from My Mexican Kitchen: Techniques and Ingredients by Diana Kennedy (Clarkson Potter, 2003)

This was very good and is a nice alternative to fresh salsa.

1 pound tomatillos, husks removed and rinsed
3 or more serrano chiles
1/2 cup loosely packed cilantro leaves
1 garlic clove, roughly chopped
2 tablespoons roughly chopped white onion
Sea salt to taste
1 tablespoon vegetable oil

Put the whole tomatillos and serrano chiles in a small saucepan, barely cover it with water, and bring to a simmer. Lower the heat and continue cooking until the tomatillos are soft but not falling apart, about 5 minutes depending on the size. Drain, reserving the cooking water.

Roughly chop the chiles, and blend all of the ingredients with about 1 cup of the cooking water until you have a fairly smooth sauce. Heat the vegetable oil in a skillet and fry the sauce over medium heat, scraping the bottom of the pan to avoid sticking. Continue cooking until the sauce has reduced and thickened to about 1 1/2 cups, approximately 10 minutes.

This sauce will keep in the refrigerator for about three days and can be frozen for about one month. It tends to separate after defrosting, so give it a whirl in the blender.

Makes about 2 cups