Melt the butter in a medium sized, heavy-bottomed saucepan. Add the leek and gently sauté without colouring until soft. In a second pan heat the milk and the cream until it is just about to boil and remove from the heat.

Add the flour to the butter and leeks and cook for two minutes stirring continuously. The flour will take on a golden sandy texture. Add the mustard.

Slowly pour in the heated cream and milk, whisking all the time, and bring to a gentle simmer. Stir regularly to prevent the mixture from sticking and burning. Add a generous grating of fresh nutmeg.

Preheat your oven to 350 F / 180 C / Gas mark 4 and boil a large pan of salted water. Cook the macaroni following the manufacturer's instructions, leaving plenty of bite as the pasta will continue to cook in the oven. Drain the pasta and set aside.

Add the truffle Cheddar and half of the Parmesan to the leek sauce and stir until the cheeses are melted. Remove from the heat. Add the thyme and check the seasoning.

Mix the pasta and sauce and pour into an oven-proof baking dish. Sprinkle with the panko breadcrumbs and the remaining Parmesan and place into the oven. Bake for thirty to thirty-five minutes until brown and bubbling. Serve with a green salad.