Trim the very ends off the leeks. Halve them and rinse out any grit. Finely slice them. Put a pan on a low heat to warm for 1 min. Add ½ tbsp oil and the leek. Season and cover. Pop on the lid. Leave to sweat for 5-8 mins while you prepare the other veg.
Peel and finely chop the onion and potato. Stir them into the slightly softened leek. Season again with a little pinch of salt and pepper. Put the lid back on the pan. Sweat the veg for 20 mins, stirring now and then till the veg are soft (they need to soften at this stage as you won't cook the watercress for long).
Peel and grate or crush the garlic. Stir it into the veg. Pour in the coconut milk. Add 500 ml boiling water. Turn up the heat and bring to the boil.
Turn the heat down a little and add the watercress, keeping a few sprigs back for garnishing. Simmer for 2 mins without the lid on the pan (this keeps the soup green). Ladle the soup into a blender and blitz, or use a hand-held blender in the pan.
Taste the soup and add more salt and pepper if you think it needs it. Ladle into warm bowls and garnish with the remaining watercress sprigs.