Heat the oil in a large saucepan or Dutch oven over medium heat. Add the onions, celery, sweet potatoes and cook, stirring occasionally, until they begin to soften, 3 to 4 minutes. Add the tomatoes and cook, breaking them up with a spoon, for 5 minutes. Add 6 cups vegetable broth or water and bring to a boil. Stir in kale, lentils, thyme, 1 ½ teaspoons salt, and ¼ teaspoon pepper. Simmer until the lentils are tender, 25 to 30 minutes. Spoon into bowls and top with the Parmesan, if using