1. In a large saucepan or stock pot, heat the oil on a medium heat then add the sliced leeks, carrots and some salt. Cook down until they are soft and sweet to taste. This should take approximately 10 minutes. After 5 minutes, add the butter and stir through.
2. Pour on the stock then bring to the boil and simmer for another 5 minutes.
3. Put the potatoes in the stock mix and simmer for a further 10 minutes. Test a potato with a knife to see if they have cooked through.
4. Take the pan off the heat and using a hand-held blender, process until smooth.
5. Serve up in bowls with crusty bread. Of course, feel free to sprinkle with croutons, fresh parsley or even a splash of cream.