Heat oven to 350 degrees. Spray 8-inch-square glass baking dish with cooking spray. In large bowl, combine oats and sugar. In medium bowl, combine milk, eggs, and vanilla; mix well. Add to oat mixture; mix well. Pour into baking dish. Bake 40 to 45 minutes or until center jiggles slightly. Remove from oven to cooling rack. Sprinkle brown sugar evenly over top of oatmeal. Using back of spoon, gently spread sugar into a thin layer across entire surface of oatmeal. Return to oven; bake just until sugar melts, about 2 to 3 minutes. Set oven to broil. Broil 3 inches from heat until sugar bubbles and browns slightly, 1 to 2 minutes. Spoon into bowls. Sprinkle with raisins and chopped walnuts and serve with milk, if desired. Makes 6-8 servings.