Preheat your oven to 400 degrees, fill a large pot with water, bring to a boil, add a generous pinch of salt, add the pasta and cook 2 minutes shy of al dente, about 7 minutes, meanwhile, make the sauce.

In a saucepan, add the butter, once melted, add the flour, stir it in cooking for about 30 seconds, then add the milk, whisking constantly to prevent lumps, then cook while stirring with a wooden spoon for about 4 to 5 minutes on medium heat or until the mixture thickens enough to coat the back of a spoon, remove from the heat, season with salt and pepper then stir in the nutmeg, mustard, 2/3 of the cheese. Mix in most of the mushrooms.

Once the pasta is cooked, drain well, place it back in the same pot, add 70% of the creamy cheese sauce, stir to make sure each piece is well coated then place in a buttered baking dish.

Top with the remaining creamy sauce, cheese and mushrooms.

Bake for 25 to 30 minutes or until golden brown and bubbly, allow to sit for 5 minutes before serving.