We love this bright, minty soup because it adds a pop of spring color to the buffet. Make it up to 48 hours ahead of time.
Melt butter in a large saucepan over medium-low heat. Add leeks, and cook, stirring occasionally, 6 to 8 minutes or until tender. Stir in chicken broth, and increase heat to high. Bring to a boil. Add peas, and cook, stirring occasionally, 3 minutes or until peas are tender. Remove from heat, and stir in mint, salt, and pepper.
Process pea mixture, in batches, in a blender or food processor until smooth. Transfer mixture to a bowl, and whisk in 1/2 cup sour cream. Season with salt and pepper, and pour into 2-oz. glasses. Chill 30 minutes to 1 hour. Whisk together lemon juice and remaining sour cream, and dollop on each serving.