1Heat oven to 375°F. In ungreased 11x7-inch (2-quart) glass baking dish, place broccoli, carrots, cauliflower and cheese sauce. Microwave uncovered on High 3 to 5 minutes, stirring once, until thawed. Stir well until cheese sauce is melted.
2Stir chicken and 3 tablespoons of the onions into vegetable-cheese mixture. Top with frozen potato nuggets.
3Bake uncovered 40 to 45 minutes or until bubbly around edges and potato nuggets are golden brown. Sprinkle with cheese and remaining 1 tablespoon onion. Bake 5 to 10 minutes longer or until cheese is melted.