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Rinse the leeks well and chop roughly.
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Heat the olive oil over medium heat.
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Add the leeks and cook for 5 minutes, stirring well.
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Add the squash and broth.
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Season with salt and pepper, stirring it in well.
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Add the dill.
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Bring to a boil and then lower to cook on simmer for 20 minutes.
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Remove from heat and let stand for 30 minutes so the flavors get a chance to meld together.
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Puree in a blender or use an immersion blender.
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Stir in the Greek yogurt and lemon zest.
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Transfer to a large, covered storage container and chill for at least 3 hours before serving.