Cook the pasta according to directions on the package.
In a pan over medium heat, make a roux: melt the lactose free butter or ghee, whisk in the flour, and continue cooking for one minute, whisking constantly, until the mixture is smooth.
Slowly add the lactose free milk, pouring with one hand while whisking with the other. The mixture, thick at first, will turn creamy and smooth.
Incorporate the Dijon mustard.
Add 3 cups of Armstrong Lactose Free Marble Cheddar Cheese and mix until a smooth and velvety consistency is achieved.
Season to taste.
When the pasta is cooked and drained, add it to the cheese sauce and stir until each pasta shell is coated with sauce.
Top with the remaining cheese and bake for 10 to 15 minutes, until the cheese is melted (continue cooking if you prefer a crispy topping).