Saute' onion in oil; add chicken broth, jalapeno peppers, salt and pepper.  Mix turkey, sour cream, American cheese and tomatoes together.  Add onion mixture to turkey mixture, along with the torn tortillas; mix well.  Place in a 13" x 9" baking pan.  Bake at 350 degrees for 30 to 40 minutes.  The last 10 minutes of baking, top with the Cheddar cheese.