Chop the watercress and the leek and blanch them for one minute in a pot of boiling salted water.
2.
Transfer them to a colander and rinse in cold water. Place them in a blender with the butter (melted) and the walnut milk and blend.
3.
In the same pot, cook the peeled and diced potatoes until tender.
4.
Mash the potatoes with a fork or a potato masher, add the watercress and leek purée and mix well until all the ingredients are incorporated. Add the extra virgin olive oil, salt and pepper.