Combine 1/2 cup mint, lemon rind, 4 teaspoons lemon juice, 1/4 teaspoon salt, pepper, and garlic in a shallow dish; add tuna, turning to coat. Cover and refrigerate 30 minutes.
Combine 1/4 teaspoon salt and water in a medium saucepan; bring to a boil. Stir in barley; cover, reduce heat, and simmer 30 minutes or until liquid is absorbed. Remove from heat; cover and let stand 5 minutes. Spoon barley into a large bowl; cool slightly. Add remaining 1/4 cup mint, tomato, green onions, capers, and olives; stir well to combine. Combine remaining 1/4 teaspoon salt, remaining 2 tablespoons lemon juice, and oil, stirring well with a whisk. Drizzle over barley mixture; toss gently to coat. Set aside.
Prepare grill or broiler.
Place tuna steaks on a grill rack or broiler pan coated with cooking spray; cook 2 minutes on each side until tuna steaks are medium-rare or until desired degree of doneness. Spoon about 1 1/2 cups barley mixture onto each of 4 plates; top each serving with 1 tuna steak.