1. Preheat broiler.
2. Bring 6 quarts water and 1 tablespoon salt to a boil. Add pasta; cook 8 minutes or until al dente; drain.
3. Combine flour and 1/2 cup milk in a saucepan over medium heat. Gradually add 1 cup milk; bring to a boil. Cook 1 minute, stirring constantly. Remove from heat; let stand 4 minutes or until it cools to 155°. Stir in 1 1/2 cups cheese. Add 1/4 teaspoon salt, onions, hot sauce, pepper, and bacon; stir. Add pasta; toss. Spoon into a 2-quart broiler-safe dish coated with cooking spray; top with 1/2 cup cheese. Broil 7 minutes.