Preheat oven to 350 degrees. Spray a 9x13 baking pan with cooking spray and set aside.
Melt butter in large skillet over medium heat. Add onion and cook for five minutes, until softened. Add in garlic and cook for 1 minute more, until fragrant.
Stir in cream cheese, green chiles, and chicken. Reduce heat to medium-low mixing constantly until cream cheese is melted and mixture is combined.
Pour a small amount of green salsa (about a 1/4 cup) into the bottom of prepared baking dish.
Divide filling evenly between 8 tortillas, about 1/3 cup each. Roll tortillas and place them seam side down in the pan. Cover enchiladas with the remaining salsa and sprinkle with cheese. Cover pan with foil and bake in the preheated oven for 25-30 minutes until cheese is melted and bubbly. Sprinkle with chopped cilantro and serve with sliced avocado.