Turn oven broiler to high while you prepare the tomatillos. Peel and wash tomatillos, then slice in half. Line a pan with foil, then add tomatillos, sliced side down. Add unpeeled garlic cloves, and whole Serranos.
Sprinkle with one tablespoon olive oil, and place under the broiler until charred (approximately five minutes).
Meanwhile, place peeled and quartered onion, cilantro, avocado, lime juice, and salt in a blender.
Carefully transfer the peeled garlic, tomatillos, Serranos, and the rendered juices into the blender or food processor. If your blender is not large enough, you can do this in batches.
Pulse until you reach the desired texture. Add a little water to thin out, if desired. Serve with tortilla chips, or as a side to any Mexican meal.
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Notes
You can adjust the spiciness of the recipe based on your taste. I use three Serranos, which are almost not spicy enough for us.
Your family may prefer one or two peppers, so adjust accordingly. Feel free to substitute jalapeños for the Serranos.
Most tomatillo salsa recipes don't call for avocados, but I find that adding avocados makes the salsa much silkier in texture, and even more tasty!
If the salsa is too thick, you can add water to thin it out while blending. Stir in ¼ cup of cold water at a time, until you reach the desired consistency.
You can store this salsa in the refrigerator in an airtight container for 3-4 days.