Heat oven to 350°F. In ungreased 1-quart microwavable casserole, mix broccoli, carrots and 1/2 cup water. Cover with microwavable plastic wrap. Microwave on High 4 to 6 minutes, stirring every 2 minutes, until vegetables are crisp-tender; drain.
Stir in sour cream, milk and cheese until well mixed. Sprinkle with onions.
Bake uncovered 15 to 20 minutes or until bubbly and vegetables are tender.