Pick the leaves from the watercress. Chop the stalks and keep the leaves separate.
Peel and finely dice the shallots and garlic.
Peel the potato and cut into small dice.
Pick the leaves from the mint. Discard the stalks and shred the leaves.
Heat a saucepan until hot and add the olive oil. Sweat the shallots for 3 minutes with a pinch of salt until they are soft and translucent. Add half of the garlic and watercress stalks and cook for a further minute. Add the potatoes and then add the stock. Bring the soup to the boil and then reduce to a simmer. Cook the soup until the potatoes are completely soft.
Whilst the soup is cooking, zest the lemon and stir it through the yogurt. Add the remaining garlic and then place the yogurt in the fridge.
Once the potatoes are cooked through, add the peas to the pan and bring back to the boil. Finally, add the watercress leaves and mint and then puree the soup in a blender. When the soup is smooth, check the seasoning and serve immediately with a dollop of the yogurt.
«The key to keeping this soup bright green and retaining all of the vitamins and nutrients is to add the peas and watercress leaves just before blending the soup.»