Combine cauliflower, carrots, celery, onion, garlic, cumin, coriander, turmeric, cinnamon, salt and pepper in a 6-quart slow cooker. Add lentils, broth and tomatoes and stir until well combined.
Cover and cook on Low 8 to 10 hours or High 4 to 5 hours or until lentils are tender.
During the last 15 minutes of cooking, stir in spinach. Just before serving, stir in lemon juice. Top with a dollop of plain Greek yogurt if desired.