Brush two 9- or 10-inch cast-iron skillets with oil.
Add half of the garlic and jalapeos to each. Cover and cook over low heat, turning the jalapeos occasionally, until they are blistered all over and the garlic is softened, about 15 minutes.
Transfer to a plate to cool. Peel the jalapeos and discard the stems, seeds and cores. Peel the garlic cloves.
Transfer the jalapeos and garlic to a blender and add the olive oil, lime juice and cilantro. Blend until smooth. Scrape the sauce into a bowl, stir in the sugar and season with salt.
Transfer the sauce to jars and refrigerate.
Make Ahead: The sauce can be refrigerated for 1 month.
