1. Peel and chop the red onion and sweet potato into small pieces.
2. In a large saucepan heat the oil and add the onion and potato.
3. Gently sweat the vegetables over a medium heat with the lid on for 10 minutes.
4. Boil the water and prepare the stock by dissolving the stock cube into it.
5. Add the stock, seasoning, spinach, watercress and rocket to the pan, bring to the boil and simmer for 5 minutes.
6. Remove from the heat and, using a blender (hand or jug), blend the soup until smooth.
8. Pour into a bowl and enjoy.