HEAT oven to 350°F.
COOK and stir onions in butter in large saucepan on medium heat 5 min. or until crisp-tender. Add 1/2 cup Cheddar and remaining ingredients; mix well.
SPOON into 1-qt. casserole.
BAKE 30 min. Top with remaining Cheddar; bake 5 min. or until melted.