heat oven to 350 ,coat a 3 quart broiler dish with nonstick cooking spray.bring a large pot lightly salted water to boiling.
cook cavatappi 8 minutes in broiling water then drain.meanwhile,in medium size saucepan cook bacon over medium heat until crisp ,6 min.transfer to paper towels. carefully pour off drippings,returning 2 tablespoons to pan ,add butter.
whisk flour until smooth,in a thin stream,whisk in milk. stir in onion flakes,salt and pepper.bring to boil over medium high heat, then reduce heat and simmer 2 min,in large bowl, toss cheddar,Colby - jack and mozzarella.
remove milk mixture from heat,whisk in American cheese and 1 1/4 cups of cheddar mixture.toss in bacon pieces with remaining shredded cheese.
in pasta pot combine cooked pasta and cheese sauce. pour half into prepared dish sprinkle with generous cup of the bacon cheese mixture.repeat layering.
bake at 350 for 20 minutes ,increase oven temperature to broil and broil 3 minutes until top is lightly browned.cool slightly before serving.