Process the pine nuts, basil, garlic and vegan cheese for the pesto in a food processor until finely chopped. With the motor running, gradually add the oil until blended. Season with salt and pepper. Set aside.
Heat the oil in a saucepan, add the leek and potato and cook over a gentle heat for 5 minutes until softened. Add the stock and bring to the boil, then cover and simmer for 10 minutes. Stir in the peas and cook for a further 5 minutes, then stir in 4 tablespoons of the pesto.
Transfer the soup in batches to the food processor or a blender and blend until smooth. Return to the pan and reheat, then season with salt and add more pesto, if liked, to taste (the remainder will keep in the refrigerator for several days). Ladle into bowls, spoon over any remaining pesto, garnish with basil leaves and serve with ciabatta croûtons.