Combine first 8 ingredients in a large bowl; stir well. Spoon mixture into an 11- x 7- x 1 1/2-inch baking dish coated with cooking spray.
Cook milk in a medium saucepan over medium heat until very hot. (Do not boil.) Stir in mashed potatoes; cook, stirring constantly, 5 minutes or until thickened. Stir in sour cream and 1/4 teaspoon salt. Spread potato mixture over vegetable mixture; sprinkle with paprika. Spray top with cooking spray.
Microwave at HIGH 30 minutes, rotating dish once, or until vegetables are tender and top is golden.