In a small skillet, cook chicken in 1 tablespoon butter until no longer pink; set aside and keep warm. Prepare mashed potatoes according to package directions.
Meanwhile, in a large saucepan, melt remaining butter over medium heat. Whisk in flour until smooth. Gradually add milk; stir in seasonings. Bring to a boil. Reduce heat; cook and stir for 1-2 minutes or thickened.
Remove from the heat. Stir in 3/4 cup Swiss cheese until melted. Add peas, corn and chicken. Transfer to a 2-qt. baking dish coated with cooking spray. Top with mashed potatoes; sprinkle with remaining cheese.
Bake, uncovered, at 350° for 25-30 minutes or until heated through. Let stand for 5 minutes before serving.