There are two general types of green enchilada sauce, says alanbarnes. The New Mexico style is made with green New Mexico chiles (Anaheims are an acceptable substitute), while most sauces from the interior of Mexico are based on tomatillos.
alanbarnes, who hails from New Mexico, makes green chile sauce by sautéing onion and garlic in a bit of oil, then adding green chiles that have been roasted, peeled, and chopped, and optionally Mexican oregano and cumin. Add chicken stock and simmer for 10 minutes, then purée (alanbarnes uses an immersion blender). Start by making it too thick, alanbarnes advises, then thin it to your desired consistency with more stock, which is easier than reducing it. "This sauce is not just for enchiladas," he notes. "Cook meat in it, use it to top eggs, ladle a bit onto a cheeseburger—the sky's the limit."
Tomatillo-based enchilada sauces tend to include onion, garlic, jalapeño, and cilantro. tecatefil makes sauce by boiling a pound of husked tomatillos in 4 cups of water until soft, and puréeing them with 2 cups of the cooking liquid, half a large onion, 4 cloves of garlic, half a bunch of cilantro, and jalapeño to taste. Heat a couple of tablespoons of lard or neutral oil, add the puréed mixture, and simmer for 5 to 10 minutes.
chef chicklet uses chicken stock to simmer the tomatillos, sautés the other ingredients, and adds them to the simmering tomatillos, puréeing it all with an immersion blender. Others prefer to roast or broil the tomatillos instead of simmering for an extra layer of flavor.