Preheat oven to 350° and cover a cookie sheet with foil
Spray chicken breasts with PAM or sprinkle with olive oil and salt/pepper
Bake chicken breasts for 30 minutes then remove from oven and let sit
Meanwhile, chop onion
Turn oven up to 400° after chicken is done
In a large skillet, melt butter over medium high heat
Sauté onion until softened, approximately 3 minutes
Shred or cut chicken into bite-sized pieces
Add chicken, tomatoes, one jar of salsa, the green chilies, and chili powder to the skillet. Add ½ teaspoon salt and pepper.
When chicken mixture comes to a boil reduce heat to low
TO ASSEMBLE ENCHILADAS:
Cover the bottom of two 11x13 baking dishes with one of the cans of enchilada sauce ( ½ of can in bottom of each)
Put baking dish on the counter by the cooktop. Put a plate nearby to work on. Take one bag of cheese and put it by the cooktop too. Open the package of enchiladas and put it by the cheese. Have two large spoons ready.
Take a separate clean non-stick skillet and heat it over medium heat
One at a time, add one tortilla to the skillet. After a few seconds (5-10), flip it over and heat the other side until it is soft and pliable.
Put the tortilla on the plate and spread about 2-3 tablespoons of the chicken mixture from the skillet in the middle of it. Sprinkle 1-2 tablespoons of cheese down the middle too. Roll up and place it seam-side down in baking pan.
Repeat with all tortillas, chicken and cheese. If there is any chicken left over, save for later or put on the top or sides of the casseroles.
Spread the other can of enchilada sauce over the two casseroles half can over each.
Spread other jar of salsa over the two casseroles half each
Spread other bag of cheese over the top of the two casseroles half each
Bake at 400° for 15-20 minutes or until the casseroles are bubbly and cheese has melted.
Serve with chopped lettuce, sour cream and sliced avocado if you like.