Melt the butter in a saucepan set over medium heat.
Whisk in the flour then cook for a minute or two until lightly golden.
Slowly whisk in the cream and milk until the sauce is smooth.
Add the nutmeg, cayenne, salt and pepper then reduce the heat and cook, stirring regularly, until the sauce is silky and thick.
Remove from the heat and stir in the grated cheese.
Peel and thinly slice the potatoes (½cm/¼inch thick).
Place a layer of potatoes into an oven-proof baking dish (approximately 30cmx20cm / 9x13in) then season with salt and pepper.
Pour over some of the cheese sauce. Repeat until all the potatoes and sauce has been used, covering the last layer of the potatoes with the remaining sauce.
Cover the baking dish tightly with foil then place in the oven and allow to bake for 45-60 minutes or until a knife can easily be inserted.
Remove from the oven, scatter over the grated Parmesan and place under the broiler for 5-7 minutes or until golden brown.
Remove from the oven, allow to cool for 10 minutes then serve.