Preheat oven to 375 degrees F. In a large saucepan melt butter on medium heat. Stir in flour, 1/2 teaspoon of the seasoned pepper, and the salt. Add broth and milk all at once. Cook and stir until thickened and bubbly. Stir in cooked chicken and thawed vegetables. Pour into two 13x9x2-inch-inch baking pans.
In a medium bowl combine mashed potatoes, Parmesan cheese, garlic, and the remaining 1/2 teaspoon seasoned pepper. Using a spoon, drop potato mixture in large mounds over chicken mixture in baking dish.
Bake, uncovered, for 30 to 40 minutes or until heated through. Let stand for 5 minutes before serving.