1. Heat a medium saucepan over medium heat. Add oil to pan; swirl to coat. Add onion; sauté 3 minutes. Add garlic and ginger; sauté 1 minute. Add curry powder and red pepper; cook 30 seconds, stirring constantly. Add broth, vinegar, and lentils. Increase heat to high; bring to a boil. Reduce heat; simmer 5 minutes.
2. Place half of lentil mixture in a blender. Remove center piece of blender lid; secure lid on blender. Place a clean towel over opening in lid. Blend until smooth. Add blended mixture, spinach, salt, and pepper to pan; stir until spinach wilts. Stir in 2 tablespoons cilantro. Serve with yogurt and remaining cilantro.