1. Cook green beans in boiling water 4 minutes or until crisp-tender. Drain and rinse with cold water; drain well.
2. Combine oil, next 6 ingredients, and 1/4 teaspoon salt in a bowl; stir with a whisk. Stir in beans, chickpeas, and tomatoes; toss. Sprinkle with walnuts, cheese, and olives.
3. Sprinkle tuna with remaining 1/4 teaspoon salt and pepper. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Add tuna to pan; cook 3 minutes on each side or until desired degree of doneness. Divide bean mixture evenly among 4 plates; top with sliced tuna.
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