Melt the oil in a large soup pot. When it shimmers, add the onion. Cook over medium heat until tender, about 10 minutes, adding the jalapeño, ginger, garlic, curry powder and salt as you prepare them, and stirring occasionally. Add the tomato paste, and cook for a few minutes, stirring, until a film forms on the bottom of the pan. Add the water, coconut milk, lentils, and sweet potato, and bring to a simmer. Reduce the heat to low to maintain a gentle simmer and cook, partially covered and stirring occasionally, until the sweet potato is tender and the lentils are soft and partially broken down, about half an hour.
Meanwhile, slice the kale off the stems. Discard the stems and tear the leaves into 1-2" pieces. Wash well, drain, and add to the pot when the soup is done. Simmer until the kale is wilted, tender and bright green, another 5 minutes or so. Remove from the heat and stir in the cilantro.
Ladle the soup into bowls, and top with a dollop of yogurt.
The soup keeps very well in the fridge for up to 5 days. It will continue to thicken, so thin it with water, coconut milk, or vegetable stock if needed.
This soup keeps very well in the refrigerator for up to a week, though the kale and cilantro will lose their verdant color. If you like, blanch the kale separately in a pot of lightly salted boiling water until bright green and crisp-tender, drain it, and run it under cool water to stop the cooking. Drain and store it separately in the fridge, and add it, with the cilantro, as you heat batches of soup (pictured here). This will keep it green and retain its texture.If you're sensitive to heat, you can leave the jalapeño out altogether.For a vegan option, omit the yogurt and finish the soup with a squeeze of lemon.As I mention above, you can probably substitute half a cup of regular coconut milk plus 1 1/2 cups of water for the light coconut milk if that's what you've got.Nutritional values are based on one of six servings.