Chop leeks and in a bowl of cold water wash leeks well. Lift leeks from water into a colander to drain. In a large saucepan cook leeks in oil with salt and pepper, to taste, over moderately low heat, stirring, until softened. Peel potatoes and cut into 1-inch cubes. Add potatoes, broth, and 2 cups water to leeks and simmer, covered, about 10 minutes, or until potato is tender. Stir in peas and simmer, uncovered, about 5 minutes, or until peas are tender.
While potato mixture is simmering, discard sorrel stems. Cut leaves crosswise into thin strips (about 6 cups loosely packed). In a blender puree potato mixture with sorrel in 4 batches until very smooth, transferring to a bowl. Whisk in yogurt and 1 cup water, adding enough water to thin soup to desired consistency.
Just before serving, stir in lemon juice and salt and pepper to taste. Garnish soup with egg and sorrel.
Recipe courtesy of Gourmet Magazine