Trim the leeks, leaving some green at the top to colour the soup. Cut in half lengthways, then cut across into 5mm (1⁄4in) slices. Rinse in a colander in plenty of cold running water. Drain well.
Peel the onion and cut it into slices about the same thickness as the leeks.
Place the pan on the hob over a medium heat and add the butter. Once the butter has melted and is foaming, add the chopped leeks and onion.
Stir to coat the vegetables in butter. Cover the pan and cook for about 10 minutes, or until soft but not brown,lifting the lid occasionally to stir.
While the leeks and onion are cooking, peel the potatoes and cut them into 5mm (1⁄4in) slices.
When the leeks and onion are ready, add the potato slices to the pan.
As soon as you have added the potatoes, pour in the hot stock, then add a little salt and pepper. Do not use too much salt because the stock may already contain some.
Add about 8 gratings of nutmeg, turn up the heat, and bring to the boil. Reduce the heat, cover, and simmer for about 10 minutes, or until the vegetables are soft.
Remove the pan from the heat and purée using an electric hand-held blender. Or, allow the soup to cool slightly and use a freestanding blender, puréeing in batches.
Bring the soup to a boil over a medium heat. Take the pan off the heat, add the cream and lemon juice, then stir and check the seasoning. Garnish with fresh herbs.
Nutritional information per serving
Saturated fat: 8g
Unsaturated fat: 4g
You can make the soup and keep it in the fridge, covered, for up to 3 days, or frozen for up to 3 months.
When leek and potato soup is served chilled, it is known as vichyssoise.
If making vichyssoise, leave to cool, then cover and chill for at least 4 hours. Chilling dulls the flavour, so check the seasoning before serving.
Mary Berry’s tips for leek and potato soup
Make sure you don’t have the heat too high, or the butter may burn.
If using a hand-held blender, keep the blades held below the level of the soup to prevent splashes, and blend for about 3 minutes.
Make sure all the ingredients are totally soft before you purée them. Any undercooked vegetables will make a lumpy soup.
Get more delicious recipe ideas by visiting out soups and stews recipe collection.
Mary Berry Cookery Course, published by DK, £17.99 (Flexi-bound edition)