1Heat oven to 350°F. Heat water, milk and butter to a rapid boil in 2-quart saucepan. Remove from heat. Stir in potatoes just until moistened. Let stand about 1 minute or until liquid is absorbed. Whip with fork until smooth; set aside.
2Meanwhile prepare vegetables as directed on package. Stir together vegetables and mushrooms in medium bowl. Spoon vegetable mixture into ungreased 1 1/2-quart casserole; sprinkle with half of the onions. Spread potatoes over onions.
3Bake uncovered 25 minutes; sprinkle with remaining onions. Bake 3 to 5 minutes longer or until mixture is hot and onions are brown.