Pre-heat oven to 190°C / 170°C fan assisted / gas mark 5 / 375°F.
To make the cheese sauce melt the butter in a saucepan then turn the heat to low and add the cassava flour. Mix together until a smooth roux has formed and keep on cooking for about two minutes.
Turn the heat up to low/medium then gradually pour the whole milk into the roux, stirring all the time until all the milk has been added and the sauce is thick and smooth.
Add the bay leaves, white pepper, salt, nutmeg and Dijon mustard. Stir these ingredients in then keep on stirring until the sauce gently starts to bubble.
Finally add in all of the cheese and stir in until melted, thick, very smooth and gently bubbling.
Remove from the heat, cover the surface of the sauce with cling film or baking parchment, then allow to rest whilst you prepare the pasta.
Bring a large saucepan of water up to a boil with ¼ teaspoon salt.
Once boiling, add the macaroni. Allow it to come to a boil again and simmer for 1 minute.
Remove from the heat, drain, then pour into a large mixing bowl.
Remove the cling film / baking parchment from the cheese sauce and whisk it well to remove any lumps which have formed during the resting time.
Pour the cheese sauce over the macaroni pasta and stir to combine them. Pour the macaroni cheese into a large baking dish.
Sprinkle the extra grated cheese topping over the macaroni cheese.
In a small bowl combine the gluten-free breadcrumbs, melted butter, salt and pepper until the butter has coated the breadcrumbs evenly.
Sprinkle the buttery breadcrumbs evenly over the top of the macaroni cheese.
Place in the oven and bake for 25 minutes.
During this time turn your grill on to pre-heat. After the macaroni cheese has finished cooking, place the dish under the grill for 5-10 minutes until the cheese and breadcrumbs have turned beautifully golden.