Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray.
Reserve 3/4 cup of the cooking sauce; set aside. In large bowl, gently mix together stuffing, turkey, vegetables and remaining cooking sauce. Spoon evenly into baking dish. Wipe out bowl with paper towel. To bowl, add potatoes and 1/4 cup of the reserved cooking sauce; stir until well mixed. Spoon potato mixture evenly over stuffing mixture; spread to cover stuffing. Sprinkle with onions.
Bake about 30 minutes or until bubbly around edges and hot in center. Heat remaining cooking sauce, and drizzle on each serving as desired.