1. Slice the potatoes, carrots, sweet potato, and leek crosswise into rounds about 1/8 in (2–3cm) thick. The potatoes can be peeled or unpeeled. Rinse the leeks well.

2. Melt the butter with the oil in a large saucepan over medium-low heat. Add the leeks and cook, stirring occasionally, for 3–4 minutes, until beginning to soften. Add the potatoes, carrots, and sweet potato, and stir for 1 minute.

3. Pour in the stock, and bring to a boil over high heat. Reduce the heat to medium-low and simmer for 20 minutes, or until the vegetables are tender but not falling apart.

4. Transfer about one-third of the vegetables to a blender or food processor with the cooking liquid. Purèe then return to the pan. Season with salt and pepper and serve hot.