Bring a pot of salted water (at least 5 cups water) to a boil and add lentils. Simmer until lentils are very tender, 30-40 minutes (depending on how old they are).
While the lentils are cooking, heat the oil in a large stockpot over medium heat and add the onion, celery, carrots, and a generous pinch of salt. Cook vegetables until soft, about 10 minutes. Add the garlic and harissa (if using) and cook a minute more, then add the chicken broth and 1 tbsp salt. Bring to a boil. When lentils are tender, add them to the soup and simmer 10 minutes. Add kale and cook until tender, about 3 minutes, then add parsley and more salt as needed.