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.