Pre-heat oven to 350 degrees F.
Bring a large pot of water plus 1 tablespoon of salt to the boil (approx 4 quarts) in preparation for the pasta.
Coat the chicken with olive oil, add salt and pepper to taste and bake in the oven for 20 minutes or until juices run clear.
Meanwhile, put two cups of water in a pot. Add tomatillos, garlic cloves and serrano peppers, and cook until tomatillos change color from bright green to olive green, approx. 5 minutes.
Drain water, but reserve 1/4 of a cup of it.
Blend tomatillos, garlic, serrano peppers, 1 teaspoon of salt, cilantro, avocado and 1/4 cup of liquid until creamy.
By now chicken should be cooked. Chop it in small cubes.
Cook the pasta according to the package instructions. Drain and toss with the sauce and chicken. Adjust salt to taste.
Top with queso fresco and serve immediately.