Combine 1 1/3 cups water, red lentils, sugar, turmeric, and broth in a medium saucepan; bring to a boil. Cover, reduce heat, and simmer 10 minutes or until lentils are very tender. Cool slightly. Place lentil mixture in a food processor or blender; process until smooth. Set aside.
Heat the oil in a large nonstick skillet over medium heat. Add the cumin seeds; cook 30 seconds or until toasted. Add onion and garlic; sauté 10 minutes or until lightly browned. Stir in pureed lentil mixture, 1 cup water, juice, salt, and red pepper; cook until thoroughly heated. Garnish with cilantro sprigs, if desired.