Sweet Potato Gratin with Mushrooms and Leeks

• 600 g (1 lb 5 oz) sweet potatoes
• 400

Peel and halve the sweet potatoes. Slice thinly. Bring the double cream and vegetable stock to a boil in a large, wide saucepan. Season with salt and pepper and add 3 sprigs of thyme. Remove the saucepan from the heat and add the sliced sweet potato. Cover and leave to absorb a little of the liquid.<