Heat oil in a medium saucepan over medium heat. Add leeks; saute, stirring occasionally, until leeks are softened and translucent, about 5 minutes. Add chicken stock, and bring mixture to a boil. Add peas, and reduce heat; simmer until peas are bright green and tender, about 5 minutes. Remove from heat.
Puree using an immersion blender, or transfer mixture to the jar of a blender, working in batches if necessary so as not to fill jar more than halfway, as hot liquid will expand when blended, and process until smooth. Season with salt and pepper.
Pour soup into serving bowls. To make polka-dots, fill pastry bag with sour cream. Pipe dots onto surface of soup, and serve.