Serves six to eight.
YOU WILL NEED
600ml/1 pint milk
600ml/1 pint cream
4 cloves garlic, crushed
Good pinch freshly grated nutmeg
1 tbsp fresh flat-leaf parsley, chopped
1 tsp fresh thyme, chopped
Knob of butter
450g/1lb potatoes, peeled
450g/1lb sweet potatoes, peeled (preferably orange-fleshed)
Salt and freshly ground white pepper
Pre-heat the oven to 160°C/325°F/ Gas Mark 3. Pour the milk and cream into a pan and add the garlic, nutmeg, parsley and thyme.
Season to taste and just heat through, but do not allow the mixture to boil, then quickly remove from the heat.
Using a mandolin cutter, thinly slice the potatoes and sweet potatoes carefully. Or use the slicing attachment of your food processor as doing them by hand can be very laborious.
Butter a large ovenproof dish. Arrange a third of the potatoes and sweet potatoes in the bottom and season to taste. Add another third of both types of the potatoes in an even layer. Season to taste.
Arrange the rest of the white and sweet potatoes on top in an attractive overlapping layer and pour over the milk mixture.
Cover the gratin with a piece of foil and bake for one hour until cooked through and lightly golden, then remove the foil and return to the oven for another 10 minutes, or until the top is bubbling and golden brown.
To serve, divide the gratin into portions and serve as required.