Add water to a pot, add tomatillos, serrano chiles, jalapeños, onion, and garlic. Bring to the boil. Cook for 3-4 minutes until tomatillos and chiles change from a bright green to military green.
Pour cooked tomatillos, chiles, garlic, and onion (without the water, but save it) into a blender. Add avocados, cilantro, salt and 1/4 cup of the cooking water you saved. Mix until well blended. Now pour the contents from the blender into a small pot or pan on the stove on warm so when you dip tortillas into it your enchiladas will be warm.
Warm tortillas on a comal or microwave. Take one tortilla, submerge it in the avocado salsa, fill with chicken, fold and top with queso fresco, repeat.