Greeny-brown Puy lentils come from the Puy region of Auvergne in France and are the best. You can buy them most cheaply dried, but they are also available cooked in packets or jars.
To cook the lentils, bring the stock to a boil in a large pan. Add the bay leaf and lentils and simmer for 25 minutes, or until the lentils are tender.
Drain. Discard the bay leaf. Transfer to a medium bowl, season well with salt and pepper and keep warm.
Split the leeks in half lengthways and wash them (see hack