HEAT oven to 425°F.
WHISK first 3 ingredients in large bowl until blended. Stir in chicken, corn and 1 cup cheese; spoon into 2-qt. casserole sprayed with cooking spray.
PREPARE cornbread batter as directed on package; stir in onions and remaining cheese. Drop large spoonfuls of batter over chicken mixture.
BAKE 20 to 25 min. or until chicken mixture is hot and bubbly, and cornbread topping is golden brown.