Heat olive oil in a Dutch oven over medium-high heat. Add onion and next 5 ingredients (onion through potatoes); cook 10 minutes, stirring occasionally. Add the bell peppers, salt, and chickpeas; saute for 5 minutes. Stir in tomato and water. Bring to a boil; partially cover, reduce heat, and simmer for 25 minutes or until the potatoes are tender. Remove vegetables with a slotted spoon. Set aside; keep warm. Reserve 1 cup of cooking liquid.
Bring reserved cooking liquid to a boil in a medium saucepan; stir in harissa, if desired. Gradually stir in couscous. Remove from heat; cover and let stand for 5 minutes. Fluff with a fork. Serve with vegetables; sprinkle with chopped parsley.