Fill a large bowl with cold water. Cut the leeks in half lengthwise and then thinly slice into half moons. Add the sliced leeks to the bowl of water, swoosh it around a little and then let them sit; the grit and dirt will fall to the bottom of the bowl.
In a large saucepan, heat the butter and olive oil. Pull the sliced leeks from the water with your hands, trying not to disturb the water too much so that the grit and dirt stays put on the bottom of the bowl. Transfer the sliced leeks to a kitchen towel to drain off some of the water then add them to the heated butter and olive oil. Add the garlic, dried basil and salt and pepper to taste. Sweat gently for 10-15 minutes till the leeks are soft and sweet. Add the beans, cream, stock, lemon juice and lemon zest. Season, if needed. Cook on high for 2-3 minutes.
Reduce the heat to a simmer. Dissolve the cornstarch in a bit of hot water and add to the soup. Add the Parmesan and simmer gently for 3-5 minutes. With a fork or potato masher, mash about 40% of the beans in the soup, leave the rest whole. Remove from heat and add the torn basil.
Reprinted with permission of Foodmonger.
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