In a large bowl, prepare cornbread mix based on instructions on the box. Spread into an 8 x 8 pan  sprayed with cooking spray.  Bake at 350 for 20 minutes.  Remove cornbread from oven and spread meat over corn bread.  Combine sour cream, 3/4 cup cheese and onion.  Spread over meat mixture.  Bake for 15 minutes longer or until heated through and cheese is melted.  Sprinkle with tomato, lettuce and remaining 1/4 cup of cheese.