Heat the oil in a large pan and add the chopped onion. Allow to soften but not brown.
Add the stock and the potato and simmer until the potato is soft. This should take about 15 minutes.
Add the lettuce, peas and fresh mint and bring back to the boil. Just allow the peas to cook through.
If serving chilled transfer the soup to a cool bowl over a bowl of ice cubes or the coldest water to cool through as quickly as possible - this way the peas don’t lose their vibrant colour.
Once cooled blend and add salt and pepper, more water if needed and lemon juice to taste.
Refrigerate as soon as cool enough for at least two hours and then serve with an ice cube and mint leaf to garnish.
You can also swirl some cream through when you serve.