Soak the cashews for 2 hours in cold water, then drain before cooking. You can skip this step if you like, but it tends to yield a creamier soup.
Heat the olive oil in a large saucepan. Then sauté the chopped leeks and celery in the oil with a pinch of salt over a medium heat for ten minutes. Keep the saucepan lid on and stir occasionally. The vegetables should be very soft, but not brown.
Remove the lid, stir in the cashews, then add the stock and bring to the boil.
Add the corn kernels, turn the heat down to a simmer and cook for ten minutes.
Remove the soup from the heat and allow it to cool for about ten minutes before pureeing with a handheld blender.
Strain the soup through a sieve into a large bowl.
Stir in the lime juice and season with salt and pepper.
Refrigerate until ready to serve. Garnish with basil leaves and serve with lime wedges on the side.