Make a small incision in the center of each serrano pepper. Place the peppers, tomatillos, onion, and two unpeeled garlic cloves on a parchment-lined baking sheet.
Drizzle with avocado oil, then roast in the oven for 15-20 minutes, or until cooked through; remove and let cool.
Blender
Transfer the roasted vegetables into a blender, peeling the garlic and removing the stems from the peppers first; then add the cilantro, raw garlic clove, avocado flesh, lime juice, salt, and water.
Blend until smooth, leaving a few specks of cilantro; then transfer to a bowl and adjust the salt to taste.