Step 1Clean the leeks. Coarsely chop the white and pale green parts of leeks. Submerge the leeks in water to remove any sandy soil. Rinse well in a colander, and shake to remove any moisture.
Step 2In a medium soup pot, heat the olive oil over medium heat. Add the leeks and cook until softened, about 5 minutes, stirring frequently. Add the squash and broth. Season lightly with one-fourth teaspoon salt and one-eighth teaspoon white pepper. Stir in the fresh dill. Bring the soup to a boil, then reduce the heat to a very gentle simmer and continue cooking, with the lid ajar, until the vegetables are tender, about 20 minutes.
Step 3Remove from heat and set the soup aside for about 30 minutes to give the flavors time to marry. Puree the soup using a blender (this will need to be done in batches, filling the blender no more than two-thirds full, and being very careful if the liquid is still hot). The soup can also be blended using an immersion blender. Add additional broth if desired to thin the soup to preferred thickness.
Step 44.Transfer the soup to a covered storage container and refrigerate until chilled, at least 3 hours, preferably overnight. This makes about 6 cups soup.
Step 5In a small bowl, combine the yogurt with the lemon zest and a pinch of salt. Serve each bowl of soup with a dollop of the yogurt mixture.