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