HEAT oven to 425ºF.
BROWN meat in large skillet; drain. Return meat to skillet; stir in water. Add Vegetables and Seasoning Mix; stir. Bring to boil; cook 1 min. Reduce heat to medium-low; simmer 5 min., stirring frequently. Stir in corn. Spoon into 2-qt. casserole sprayed with cooking spray; cover with Cheese Sauce.
MICROWAVE Mashed Potatoes, milk and butter in microwaveable bowl on HIGH 3 min.; stir. Spoon over cheese sauce.
BAKE 18 to 20 min. or until filling is heated through and topping is golden brown.