Make the croûtons. Brush the bread cubes with the oil and spread out in a roasting tin. Sprinkle with the sesame seeds and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 10–15 minutes, or until golden.
Meanwhile, melt the butter in a large saucepan, add the onion and cook gently for 5 minutes, or until softened. Add the peas, lettuce, stock, lemon rind and juice and salt and pepper to taste. Bring to the boil, then reduce the heat, cover and simmer for 10–15 minutes.
Leave 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 reheat gently. Spoon into warmed serving bowls and sprinkle with the croûtons.