Grill the tomatillos, onion, garlic, jalapeno/serrano chiles, on a cast iron grill pan or heavy skillet; until nice grill marks about 5-8 minutes on medium-high heat.
Add all the grilled veggies, avocado, salt, and lime juice (except cilantro) to a high-speed blender or food processor. Blend or pulse until completely smooth and combined will be a creamy smooth texture. Scrape down the sides as you blend. Add cilantro leaves last, blend quickly.
Store in fridge for up to 5 days in a sealed mason jar.
Serve with tacos, taquitos, nachos, quesadillas, or burritos. Or serve with tortilla chips as a dip!