Preparation
1. In a large skillet, heat the oil over a medium heat. Add the onion and celery and cook, stirring often, until just softened, about 5 minutes. Transfer to a slow cooker, along with the lentils, spices and lemon juice. Add the broth and enough water to come about 3 inches above the vegetables. Cover and cook on HIGH for 1 hour.
2. Turn the cooker to LOW and cook the soup for 5 to 6 hours. Season with salt and pepper, if desired. Add water to thin if the soup is too thick.