1. Brush two 9- or 10-inch cast-iron skillets with oil. Add half of the garlic and jalapeños to each. Cover and cook over low heat, turning the jalapeños occasionally, until they are blistered all over and the garlic is softened, about 15 minutes. Transfer to a plate to cool. Peel the jalapeños and discard the stems, seeds and cores. Peel the garlic cloves.
2. Transfer the jalapeños 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.