Cook the macaroni 3 minutes less then the package directions. Drain and stir in olive oil so the noodles don't stick. Set aside.
In a small bowl beat the egg. Set to the side.
To a large pot add the butter and let it melt. Whisk in the flour until it turns golden brown. Whisk in the milk, and ground mustard. Whisk frequently for about 5 minutes or until it starts to thicken. Remove 1/4 of the mixture and whisk quickly into the beaten egg until well combined. Whisk the egg mixture into the sauce in the pot. Stir in 4 cups of the cheese once melted stir in the macaroni, seasoned salt, salt, and pepper.
Pour into a greased 9x13 baking dish. Sprinkle with remaining 1 cup of cheese. Bake for 20 minutes or until the cheese on the top is melty. Enjoy!