• 600 g (1 lb 5 oz) sweet potatoes
• 400 g (14 oz) double cream (heavy cream) (alternatively crème fraîche)
• 200 ml (7 fl oz) vegetable stock
• Salt, pepper
• 6 sprigs thyme
• 250 g (9 oz) mixed mushrooms
• 1 small leek
• 1 tbsp olive oil
• 1 tbsp butter
• About 80 g (2¾ oz) parmesan, grated

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.

Trim and slice the mushrooms. Trim the leek, wash thoroughly and slice into rings. Pick the leaves off the remaining sprigs of thyme. Heat the olive oil and butter in a large pan. Add the mushrooms and fry for about 5 minutes. Season with salt and pepper and stir in the picked-off thyme leaves and leek.

Preheat the oven to 190°C (375°F/Gas mark 5). Arrange the vegetables in a large baking dish, alternating between layers of sweet potatoes and the mushroom and leek mixture. Season with salt and pepper, pour over the remaining liquid and sprinkle with parmesan.

Transfer the dish to the oven and bake for about 30–40 minutes (depending on the thickness of the sweet potato slices). If you like a crunchy top, switch to the oven’s grill function towards the end and grill until golden brown.