Step 1 Bring a medium saucepan of water to a boil. Add the chiles and cook gently until tender, 15 to 20 minutes. Drain well. In a separate pan, combine the broth, garlic and onion. Bring to a boil, add the drained chiles and simmer briskly until the onion is tender, about 10 minutes. Remove from the heat and set aside to cool about 10 minutes. Then, working in batches if necessary, purée in a blender. Set aside in a bowl. The sauce can be made up to 3 days in advance; cover and refrigerate until ready to use if making ahead of time.