Watercress soup

Ingredients

• 1 tablespoon vegetable oil
• 2 medium leeks, trimmed and sliced
• 2 large, floury baking potatoes (such as King Edwards or maris pipers), trimmed and diced
• ¼ teaspoon smoked paprika
• 80g watercress
• 500ml reduced-salt vegetable stock
• Freshly ground black pepper, to taste
• 250ml skimmed milk