Combine onions, carrots, garlic, oil, cumin, coriander, turmeric, cinnamon and pepper in a 5- to 6-quart slow cooker (see Tip). Add broth, water, cauliflower, lentils, tomatoes and tomato paste and stir until well combined.
Cover and cook until the lentils are tender, 4 to 5 hours on High or 8 to 10 hours on Low.
During the last 30 minutes of cooking, stir in spinach. Just before serving, stir in cilantro and lemon juice.
