In 3-quart saucepan, cook and drain macaroni as directed on package. Return to saucepan; stir in sour cream. Cover to keep warm.
Meanwhile, heat oven to 350°F. Spray 8-inch square (2-quart) glass baking dish with cooking spray. In 2-quart saucepan, stir milk, onion, flour, mustard, seasoned salt, red pepper and black pepper with wire whisk until smooth. Cook over medium heat, stirring constantly, until mixture boils and thickens. Remove from heat. Stir in cheese until melted.
Add cheese sauce to cooked macaroni mixture; mix well. Spoon into baking dish. In small bowl, mix topping ingredients until crumbly. Sprinkle over casserole.
Bake 20 to 25 minutes or until edges are bubbly.