Place the cauliflower, broccoli, and carrots in a steamer basket; place in a large sauce pan over 1 inch of water. Bring to a boil; cover and steam for 7-9 minutes or until crisp tender. Meanwhile, in a large skillet, sauté mushrooms and onions in butter until tender.
Drain vegetables. In a large bowl, combine the soup, milk and cheese sauce. Add the vegetables and mushroom mixture; toss to coat. Transfer to a greased 2 quart baking dish. Cover and refrigerate overnight.
Remove from refrigerator 30 minutes before baking. Bake, uncovered, at 350°F for 40-45 minutes or until bubbly.