To the skillet, add remaining 1 teaspoon oil, and char the outside of poblano and serrano peppers until they are slightly blackened on all sides, about 10–12 minutes. Remove peppers from skillet, and turn off heat. Place poblano peppers in a zip-top bag until cool enough to handle, about 10–15 minutes. Peel, seed and chop poblano peppers; add to blender. Cut stem off serrano pepper, and add to blender. Add garlic, cilantro and ½ teaspoon salt to the blender. Puree until smooth. Taste, and season with salt and pepper as desired.