Heat 1 tablespoon Olive oil and 1 tablespoon Butter in a large pan. Add 2 Shallots and 1 rib Celery and cook on low until softened - about 5 minutes.
Add 150 g Potato and 300 ml Vegetable stock. Simmer for 10 minutes.
Add 160 g Watercress and cook for 3 minutes.
Blend until smooth. Add 60 ml Whole milk and 20 ml Cream, stir and serve.
Notes
Use a stick blender to blend this soup. If you are going to use a jug blender, make sure it cools down first, as putting hot soup in can cause it to break.
If you are making this soup for young children, then use a low salt stock.
You may need to add a little bit more stock to the soup after blending, depending on the consistency that you want. If you have cooked the soup on too high a heat, then it will need more stock otherwise it will a little thick. It is meant to be a thinner soup.