Wash lettuce, dry thoroughly and chill until crisp and ready to serve.
To make Dressing, process egg in a food processor with vinegar. With motor still running, add oil in a slow stream until thickened. Transfer to a jug and stir in gherkin. Season to taste.
Cut lettuce in half, then cut each half across into six wedges. Arrange on a large serving platter and spoon over Dressing. Scatter with green onion, sliced radish and parsley to serve.