Heat oven to 350ºF. In 2-quart saucepan, heat water, milk and butter to boiling. Remove from heat. Stir in 1 pouch potato mix and 1 pouch seasoning just until moistened. Let stand about 1 minute or until liquid is absorbed. Whip with fork until smooth; set aside.
In large skillet, heat oil over medium heat. Cook mushrooms and onion in oil, stirring occasionally, until tender. Add frozen vegetables and cooking sauce. Heat to boiling; reduce heat. Simmer uncovered 5 minutes.
Pour mixture into ungreased 1 1/2-quart casserole. Spread mashed potatoes over top.
Bake uncovered 25 to 30 minutes or until filling is hot and potatoes are light golden brown.