Preheat oven to broil. Roast the tomatillos, serrano peppers, jalapeños peppers, and garlic on a foil-lined baking sheet until blackened all over, about 5 minutes. Flip them over and roast the other sides for an additional 4 to 5 minutes. Watch carefully to prevent the tomatillos from burning.
Place roasted peppers in a plastic bag, close the bag and allow the peppers to steam until the skins loosen, about 10 minutes.
While the peppers are cooling off, peel off skin from garlic and discard. After the peppers have steamed, peel skin off and discard.
Place in a blender the cooled tomatillos, roasted and peeled peppers, roasted garlic, avocados, cilantro, water, and salt. Blend until creamy.
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