Place the onion, garlic, ginger, sweet potato, lentils, broth, coconut milk, chili paste, cumin, and garam masala into a slow cooker. Stir a few times to mix everything well. Cover and cook 5-6 hours on low, or 3-4 hours on high, until lentils and sweet potatoes are tender.
Stir in the lime juice and tomatoes. Allow to cook for about 10 minutes more, just to bring the temperature back up. Season with salt and pepper to taste. Ladle into bowls and top with cilantro. Serve.