Remove the rough outer layer of the leeks, slice the leeks lengthways and open each leek like a fan, let water run through the layers for a minute or two to get rid of any soil. Then slice the leeks.
Heat the butter in a large saucepan, add the onion and leeks, cook for about 5 minutes until they are soft. Add the potato and garlic and cook for another minute. Add the stock, season with black pepper, bring to the boil and then simmer for 15 minutes or until the potato is cooked.
Use a hand blender to make a smooth soup, or leave it if you prefer it chunky.
Top with grated parmesan and black pepper if you wish.