1. Heat oil in a saucepan. Cook onion for 2-3 minutes or until soft. Add stock, potato, lettuce, peas and mint and simmer for 20 minutes or until potato is tender. Season with pepper to taste. Set aside to cool.
2. Transfer soup mixture in batches to the bowl of a food processor or blender. Process until smooth. Chill for at least 2 hours before serving.
3. To serve, garnish with a tablespoon of yogurt, swirled through each bowl of soup.