Make the croûtons. Brush the bread cubes with the oil and put them in a roasting tin. Sprinkle with the sesame seeds and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 10 minutes or until golden.
Meanwhile, heat the butter in a large saucepan, add the onion and cook for 5 minutes or until beginning to soften. Add the peas, lettuces, stock, lemon rind and juice and seasoning. Bring to the boil, reduce the heat, cover and simmer for 10 minutes.
Allow the soup to cool slightly, then transfer to a food processor or blender and process until smooth. Return the soup to the pan, adjust the seasoning if necessary and heat through gently.
Spoon the soup into warm serving bowls and serve sprinkled with the croûtons.