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Heat the oil in a large saucepan over a medium heat. Add the leeks and garlic and fry for 7-10 mins or until softened and translucent.
Pour in the hot stock and simmer for 5-10 mins. Stir through the watercress, reserving a few leaves for garnish, then the peas, and cook for 5 mins until wilted. Use a hand blender or processor and whizz until smooth. Stir through the lemon juice and zest, then season to taste. Stir through half the parsley. Ladle into bowls and top with the remaining parsley, reserved watercress and a drizzle of olive oil. Swirl through some crème fraîche, then serve with crusty bread, if you like.