Place the red lentils, sweet potatoes, onion, and garlic in the slow cooker.
Add the curry powder, turmeric, and cumin, then stir to coat the vegetables and lentils.
Pour in the coconut milk, diced tomatoes, and vegetable broth. Add salt and pepper to taste and stir everything together.
Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours, until the lentils and sweet potatoes are soft.
Stir the curry well, then serve over rice and top with fresh cilantro.