Heat 4 tablespoons filtered/bottled water in a heavy-based frying pan over a medium heat. Add the onion and steam-fry for 2 minutes, then stir in the garlic and steam-fry for 30 seconds, stirring frequently, and adding a little extra water if needed. Add the sweet potato and stir-fry for another 3–4 minutes then pour in 500ml/17fl oz/2 cups boiling filtered/bottled water. Cover with a lid and leave to simmer for 10–15 minutes until the sweet potato is tender. Add the watercress, then either transfer to a blender or food processor, or use a stick blender, and blend – either until smooth and creamy or until roughly blended, depending on how you like it. Serve hot.
Recipe by Grace Cheetham at https://www.gracecheetham.com/gluten-free-dairy-free-detox-watercress-sweet-potato-soup