Place sweet potato, 3 cups water, and next 7 ingredients (through turmeric) in a 5- to 6-quart electric slow cooker coated with cooking spray. Add 1 teaspoon sugar, salt, garlic, and tomato paste; stir well to combine. Cover and cook on low 8 hours. Turn off heat; stir in coconut milk. Let stand, covered, 5 minutes. Combine remaining 1/2 cup water, remaining 2 teaspoons sugar, and vinegar in a microwave-safe bowl; microwave at high 2 minutes or until boiling. Add onion to vinegar mixture; let stand 20 minutes at room temperature. Drain. Place 1/2 cup rice in each of 8 shallow bowls. Top each serving with 1 1/4 cups lentil mixture, 2 tablespoons red onion mixture, and 2 tablespoons cilantro.