Wash and dry the watercress thoroughly. Set some pieces aside for garnish for the finished soup.

Peel the potatoes, wash them and cut them into very small pieces.

In a medium-sized saucepan over medium heat, melt the butter.

Add the watercress and potato pieces to the melted butter. Stir to coat.

Add the salt and white pepper and stir again.

Cover the saucepan, reduce the heat to the lowest possible setting and let the mixture cook for 5 minutes.

Add the fresh heavy cream and the vegetable stock. Mix everything to combine.

Cook the soup for another 10-15 minutes or until the potato pieces are softened.

Remove the soup from the heat.

Transfer the soup to a food processor or blender, add the freshly squeezed lemon juice and fresh lemon zest and purée.

Transfer the soup to bowls, garnish with the reserved watercress and serve.