Boil and cook pasta accordingly in a large stock pot.
In a large sauce pot, melt butter over low-medium heat. Whisk in flour until it becomes a paste mixture.
Pour and whisk milk in about ½ cup at a time. Let mixture thicken and then pour more milk in.
Add all seasonings and 5 cups of mozzarella. Stir mixture until cheese has melted. Do not lower heat too much because the cheese will start to cool and clump together.
Preheat oven to 400 °F.
In a greased 8x13 casserole dish, pour in drained cooked pasta, toss in cooked chicken, and pour hot cheese mixture on top and stir all together. Lay sliced tomatoes over top and sprinkle last 1 cup mozzarella cheese of the very top.
Bake uncovered for 20 minutes or once macaroni mixture is bubbling hot and cheese on top is melted.
Take out of oven and let rest for 10 minutes. Cut, serve, and enjoy.