Cut tops off peppers and remove seeds. In a large kettle, cook peppers in boiling water for 3-5 minutes or until crisp-tender. Drain and rinse in cold water; set aside.
In a nonstick skillet coated with cooking spray, saute the onion, mushrooms, celery, apple and garlic for 4-5 minutes or until vegetables are tender. Remove from the heat and set aside.
In a saucepan, combine the broth, salt, cumin, pepper and turmeric if desired. Bring to a boil; stir in couscous and oil. Remove from the heat; cover and let stand for 5 minutes. Fluff with a fork. Stir in the vegetable mixture, raisins, pine nuts and parsley. Spoon into peppers.
Place in an 8-in. square baking dish coated with cooking spray. Cover and bake at 350° for 15 minutes or until heated through.