This sauce made with tomatillos, the small green tomatoes that come in a husk, is a classic Mexican sauce used for many dishes including Lengua en Salsa Verde, Carne en Su Jugo, Carnitas Tacos, Huevos Divorciados and many others.
Roast the tomatoes and the chiles.
In a molcajete (or in a blender) grind the garlic, serrano peppers, salt; add the chopped tomatoes and the onion until they incorporate well with the rest of the ingredients.
Pour in a saucer and sprinkle finely chopped cilantro.
For red sauce substitute green tomatoes with red tomatoes.