Puree chilies in blender, adding a little water if the puree seems dry.
Cook garlic and tomatillo in a large dry cast iron skillet over med-hi heat, turning occasionally, until golden brown and softened, about 8 minutes. Peel garlic. Coarsely chop garlic and tomatillos; set aside.
Heat oil in the same skillet over medium heat but not smoking. Cook onion until softened and translucent, about 5 minutes. Add chili puree, garlic, tomatillos, 1/2 cup water, oregano and cumin. Bring to a boil; reduce heat. Simmer until sauce is thickened, about 15 minutes. Season with salt. If not serving immediately, refrigerate up to 2 days. Just before serving, heat over low heat; add water if sauce seems too thick.