Preheat oven to 350 degrees. Cook macaroni according to package directions; rinse with cool water and drain.
Meanwhile, melt butter in large saucepan over medium heat. Add flour and stir to mix well. Cook for 2-3 minutes.
Gradually whisk in half and half and simmer for 3-5 minutes, stirring constantly.
Stir in heavy cream, cream cheese, Dijon mustard, fresh thyme, onion powder, nutmeg, and cayenne pepper. Season with salt and pepper, to taste.
Pour pasta into a large bowl. Add half of the Fontina cheese, the white American cheese and the smoked cheddar. Toss to mix well.
Stir the sauce into the pasta and cheese.
Pour the pasta into a greased 9×13 inch baking dish. Top with remaining Fontina cheese.
Bake for 25-30 minutes or until golden brown. Let stand 10 minutes before serving.