Put the water, onion, and lentils in a large soup pot and bring to a boil over medium-high heat. Decrease the heat to medium-low, cover, and simmer until the lentils are tender, about 20 minutes. Add the kale, tomatoes, curry paste, and salt and stir to combine. Cook until the kale is tender, about 5 minutes. Season with pepper and additional curry paste to taste.