Place black lentils in a microwave-safe bowl and cover with 4 cups water. Microwave until softened, about 5 minutes. Stir and add more water if needed. Microwave for 5 minutes more. Drain and rinse.
Combine vegetable broth, tomatoes, carrots, onion, turnips, green bell pepper, butter, ginger, garlic, coriander, cumin, and salt in a large slow cooker. Stir in lentils.
Cook on High until carrots are tender, 4 to 6 hours.