Use chiles called Big Jims--long, green chiles (also known as New Mexican chiles) that are very common in the Southwest. If you can't find them, use Anaheims or another mild chile such as a poblano. If you want the sauce spicier, add a diced jalapeño or serrano chile.
Preheat oven to 450°.
Place chiles on a foil-lined baking sheet; bake at 450° for 20 minutes or until blackened, turning once. Place chiles in a zip-top plastic bag; seal and let stand 10 minutes. Peel; discard seeds and membranes. Cut chiles into thin strips. Place chiles and remaining ingredients in a medium saucepan. Bring to a boil; cover, reduce heat, and simmer 20 minutes.