Preheat oven to 400 degrees. Oil 9x13 pan and place chicken breast in it. Season chicken with salt, pepper and garlic powder (i add cumin and chili pwd, too). Bake for 20-30 minutes, or until fully cooked. Remove and let cool to the touch, then chop into small dice.
Reduce oven temperature to 350 degrees.
While chicken is baking, in med-large non-reactive bowl(not metal) mix soup, sour cream, diced onions, drained chiles and seasonings well. Add cooled chicken and mix well. Cover and refridgerate if you are not ready to assemble.
I use a large jellyroll pan, but you can use whatever they will fit in. Coat pan with light colored oil, place a few tortillas on oiled pan, spread about 4 tablespoons across each tortilla, sprinkle a little mexican cheese blend, roll them up and repeat...and repeat...and repeat!
Once complete, brush all enchiladas with light colored oil and bake at 350 degrees for about 30 minutes. Remove from oven and brush with enchilada sauce, spread remaining mexican cheese and cheddar cheese and top with your favorite toppings. Return to oven until cheese is melted and gooey.
Serve with sour cream, guacamole, extra enchilada sauce or whatever! Enjoy!