Combine yogurt and cilantro in beaker of an immersion blender; blend until cilantro is finely chopped. Cover yogurt sauce, and chill.
Heat oil in a large saucepan over medium heat. Add onion, bell pepper, and garlic; sauté 8 minutes or until tender. Add curry; cook 1 minute, stirring constantly. Add broth and tomatoes to pan; bring to a boil. Stir in lentils. Cover, reduce heat, and simmer 30 minutes or until lentils are very soft. Stir in ginger.
Using an immersion blender in pan, coarsely puree lentil mixture. Stir in red pepper. Serve lentil mixture with yogurt sauce.