In a large saucepan, heat oil over medium heat. Add bell pepper, onion and garlic; cook, stirring occasionally, until softened but not browned, 10 to 12 minutes. Add tomatoes, onion and reserved tomato juice (from Quick-Roasted Chicken) and diced tomatoes; simmer 5 minutes. Stir in chicken, chickpeas, olives and parsley. Cook until heated through, about 5 minutes (if sauce is too thick, add up to 1/2 cup water); salt and pepper to taste. Serve over couscous and garnish with parsley, if desired.