1Heat oven to 400°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray. Sprinkle chicken with seasoned salt. In 12-inch nonstick skillet, cook chicken and onion over medium-high heat 4 to 6 minutes, stirring occasionally, until chicken is brown.
2Stir in gravy. Heat to boiling. Reduce heat to medium-low; cover and cook about 5 minutes, stirring occasionally, until chicken is no longer pink in center. Meanwhile, place vegetables in colander. Rinse with hot water until thawed. Remove chicken mixture from heat. Stir in vegetables and sour cream; keep warm.
3In medium bowl, mix cornbread and muffin mix, milk, butter and egg with spoon just until moistened (batter will be lumpy). Spoon chicken mixture into baking dish. Drop batter by spoonfuls around edges of warm chicken mixture. Sprinkle cheese over batter.
4Bake uncovered 20 to 22 minutes or until cornbread is deep golden brown. Let stand 5 minutes before serving.