Once cooked, leeks are rendered sweeter and softer than the average onion – a base of buttery leeks will add subtle depth to any soup, stew or risotto. Choose ones on the small or slender side for most recipes – large leeks fare better in blended soups. Any slightly tough outer layers can be peeled away and used to make vegetable stock. Most leeks need a rinse to remove earth left between the layers; if slicing the leeks, just rinse the slices in a colander.