In a large saucepan cook leeks in oil with salt and pepper to taste over moderately low heat, stirring, until softened. Peel potato and cut into 1-inch cubes. Add potato, broth, and 1 cup 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.
In a blender puree potato mixture with sorrel in 2 batches until very smooth, transferring to a bowl. Whisk in sour cream and remaining 1/2 cup water, adding additional to thin soup to desired consistency. Chill soup, covered, at least 2 hours, and up to 24.
Just before serving, stir in lemon juice and salt and pepper to taste. Garnish soup with egg and sorrel.