PREPARATION:
- Place beans in a large saucepan; cover with 4 inches of water. Bring to a boil over high heat. Uncover pan; boil for 2 minutes. Remove pan from heat; cover. Let stand 1 hour in water, then rinse and drain.
- Combine oil, basil, vinegar, mustard, lemon juice, salt and pepper in glass jar with tight-fitting lid. Cover. Shake well; set aside.
- Rinse tuna; pat dry with paper towels. Place tuna in shallow glass dish. Pour 3/4 cup oil mixture over tuna. Coat tuna in mixture; cover and marinate in refrigerator 30 minutes.
- Prepare grill for direct cooking. Transfer beans to large bowl. Add corn and tomatoes. Stir in remaining oil mixture. Cover; marinate at room temperature until ready to serve.
- Drain tuna; discard marinade. Place tuna on grid. Grill over medium-high heat 6 to 8 minutes or until tuna flakes easily when tested with fork, turning halfway through grilling time.
- Arrange tuna on arugula-lined plates. Spoon bean mixture over tuna. Garnish with lemon slices and dill, if desired.
