Spray an 11x7" casserole dish with non stick cooking spray, and set aside.
Cook the pasta according to the package instructions. Drain, run under cold water to stop cooking, and set aside.
In a large skillet, over medium heat, melt the butter. Stir in the garlic cooking just until fragrant, about 30-45 seconds. Don't over cook and let the garlic burn or turn bitter.
Whisk in the flour and pepper until smooth, and cook until the mixture's bubbling. Whisk in the half & half, a little at a time, until completely incorporated and simmer until the sauce is smooth and slightly thickened. Remove the skillet from heat.
Stir in 3/4 cups of both the cheddar and mozzarella cheeses, and the Parmesan cheese, whisking until completely melted and the cheese sauce is smooth.
Add the pasta & chicken to the sauce, stirring gently until everything's evenly combined and generously coated in the cheese sauce.
Spread the mixture out evenly into the prepared casserole dish. Sprinkle the remaining cheese evenly out overtop. You can be a little generous and add a bit extra for added cheese effect.
Bake the dish at 350 degrees for 25-30 minutes, or until the dish's bubbly throughout and the cheeses are melted and golden brown at the edges. Let rest for 2-3 minutes before scooping and serving.