Heat the oil in a large saucepan. Sweat the onion and garlic for 3–4 minutes until softened. Add the potato and cook for a further 2–3 minutes.
Stir in the stock, bring to the boil and leave to simmer for 5 minutes.
Add the cucumber to the saucepan, and cook for a further 3 minutes or until the potatoes are tender.
Add the watercress and allow to wilt, then transfer the mixture to a blender or food processor and purée until smooth.
Bring a small saucepan of water to the boil. Cook the beans for 3–4 minutes until tender. Add the beans to the soup, season and warm through.