For the watercress soup, heat the olive oil in a pot and sauté the onions until translucent. Pour in the vegetable soup and add the peeled and diced potatoes.
Cook on low heat for 10 minutes. Finely chop the cress (put a few leaves aside for decoration) and add to the soup.
Bring to the boil again. Then remove the pot from the heat, puree the soup with a blender, add salt and pepper, a dash of Oat Cuisine and blend again.
Serve with the plucked leaves.