Combine onions, carrots, garlic, oil, cumin, coriander, turmeric, cinnamon and pepper in a 5- to 6-quart slow cooker. 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.
Add spinach to the slow cooker. Stir, cover and cook on High for 30 minutes.
Just before serving, stir in cilantro and lemon juice.