Roughly chopped fresh cilantro and chopped toasted cashews, for serving
Heat the fat in a medium saucepan over medium heat. Add the onion and scallion and cook, stirring, until tender, 3 to 5 minutes. Add the potato, ginger, garlic, 1/2 teaspoon salt and a few grinds of pepper, and cook, stirring, until the potatoes are well coated, 2 to 3 minutes more. Pour in the broth and bring to a simmer.
Simmer until the potatoes are tender, about 20 minutes, adjusting the heat as needed to maintain a simmer. Add the kale and watercress and simmer 5 minutes more. Remove from the heat and let cool at least 5 minutes. Carefully puree until smooth with a handheld immersion blender or in 2 to 3 batches in a blender (do not fill the blender more than halfway full per batch).
Stir in the coconut milk and reheat the soup if necessary, adjusting the consistency and seasoning with water, salt and pepper.
Serve hot sprinkled with the cilantro and cashews.
When blending hot liquid, first let it cool for 5 minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters and pulse until smooth.
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