Cook the peas in boiling water for 3 mins. Drain and cool with cold running water. Put the peas, basil, lettuce, vegetable stock and 3⁄4 of the crème fraîche together in a blender and process until smooth. Do this in two batches. Season, cover and chill for at least 1 hr before serving. Add a swirl of remaining crème fraîche and serve.