Preheat oven to 375°F. In a small bowl, stir together soup, 1/2 cup cheddar, and sour cream. In a large fry pan, cook onions, chilis and chili powder in butter until tender. Stir in chicken and 1/2 of the soup mixture.
Remove from heat.
Spread about 1/4 cup along centre of tortillas and roll and place seam-side down in a greased 9×13 inch casserole. Spread remaining soup mixture over enchiladas. Pour pasta sauce or salsa over that, and spread evenly.
Cover with foil and bake for 30 minutes. Remove foil and sprinkle with remaining cheddar and bake for 5 more minutes or until cheese melts.