Preheat oven to 325 degrees In a large skillet, saute onions in butter. Drain all tomatoes and reserve juice. To the onions, add the drained tomatoes and chiles, tomatoes, cheese, chicken and sour cream. Line bottom of a greased 9x13 inch baking dish with the tortilla pieces. Top with the chicken mixture and bake uncovered for 45 minutes. Add small amounts of tomato liquid if mixture seems too dry. (I like this a bit gooey, so I add tomato liquid to suit the texture I like.) Serves 12 15.
P.S. I use fresh but cold corn tortillas instead of stale, and I don't even bother to deep fry them. I just break them up into bite size pieces