Preheat the oven to 400° F and lightly grease a 9" x 13” baking dish (a 3 or 4 quart pan will work as well) with non-stick cooking spray. 

Bring a large pot of salted water to a boil and cook the cavatappi just until al dente, according to the package directions. Reserve ½ cup of pasta water before draining. Set aside. 

Heat a large Dutch oven or pot over medium heat. Add the bacon and cook, stirring often, for 5-7 minutes, or until crisp and browned. Remove the bacon to a paper towel-lined plate using a slotted spoon and leave the grease in the pan. 

Season the chicken pieces with 1 tablespoon of the ranch seasoning. Add the chicken to the pan with the bacon drippings and sear for 2-3 minutes per side, or until the chicken is lightly browned – it won’t be fully cooked yet, that’s okay! Remove the chicken to a plate and set it aside. 

Discard all but 2 tablespoons of the drippings (no need to wash the pan). Add the diced onion to the drippings and cook for 3-5 minutes, or until tender. 

Add the garlic and flour. Cook for an additional minute, stirring often. 

Next stir in the milk and cream, making sure to scrape any browned bits from the bottom of the pan. Use a whisk to smooth out any lumps. 

Bring the mixture to a simmer and cook for 1-2 minutes, or until bubbly and just barely thickened. It will thicken much more when the cheese is added. 

Add 1 cup of the shredded mozzarella, 1 cup of the grated parmesan, all of the cheddar, and the remaining tablespoon of ranch seasoning (and the red pepper flakes, if using). Stir until the cheese has completely melted. 

Remove the pan from the heat and stir in the pasta, chicken, and bacon. 

Transfer the pasta to the greased baking dish and then sprinkle over the remaining mozzarella and parmesan. 

Bake uncovered for 20 minutes, or until the cheese is melted and lightly browned. 

Garnish with sliced green onions and serve immediately.