Preheat oven to 400°F. Spray a 13 x 9 inch baking dish with cooking spray. Set aside.
In a medium-size mixing bowl whisk together cream, half and half, salt, pepper, garlic, onion powder, Dijon and nutmeg until fully blended.
Mix cheddar and Colby jack cheese together.
To assemble: Layer 1/3 sliced potatoes on bottom of dish. Drizzle with 1/3 cream sauce, 1/3 shredded cheese and 1/3 Parmesan. Repeat two more times ending with shredded cheese and Parmesan. (Season each layer of potatoes lightly with salt and pepper to your taste)
Cover tightly with foil. Bake for 40 minutes. Lower oven temperature to 350°F. Continue to bake uncovered for another 35-40 minutes or until potatoes are fork tender.
Allow to stand on counter for 10 minutes, then serve garnished with parsley, if desired.