Preheat oven to 350 degrees F.
Butter a 9x13 baking dish and set aside.
Cook the pasta so that it is still firm and not soft (it will finish cooking in the oven)
Drain pasta and place in buttered baking dish.
In a large saucepan melt the butter on medium high heat
Add arrowroot powder and allow to cook for 1-2 minutes, stirring continuously.
Stir in the milk, sea salt, black pepper. dry mustard and minced onion.
Bring that mixture to boiling and allow to thicken, stirring frequently.
Add 3 1/2 cups of shredded cheese to the thickened milk mixture.
Stir the mixture until the cheese has fully melted.
Pour the cheese mixture over the pasta in the 9x13 baking dish.
Sprinkle remaining cheese on top of the pasta.
Bake for 30 minutes.
Allow the macaroni and cheese to sit for 5-10 minutes before serving.