Preheat the oven to 160ºC.
Cut the potatoes into thin slices without cutting all the way through them. The incisions should go about three-quarters of the way down.
Place the potatoes in a ceramic tray. Sprinkle with salt and scatter the thyme and rosemary.
In a saucepan, place the garlic, mustard and cream. Bring the mixture to the boil over medium heat and pour over the potatoes. add the grated cheese.
Place in the oven loosely covered with parchment paper and foil.
Roast for about 90 minutes, basting from time to time. Remove from the oven and enjoy while hot and crispy.