Combine first 5 ingredients in a small jar; cover tightly, and shake vigorously. Set aside.
Wash beans; remove ends. Arrange beans in a vegetable steamer over boiling water. Cover and steam 5 minutes or until crisp-tender. Drain.
Wash potatoes. Cook in boiling water to cover 20 minutes or just until tender. Drain and cool slightly. Cut potatoes into 1/4-inch-thick slices.
Combine green beans, potato, sweet red pepper, and onion; toss gently. Add half of vinegar mixture; toss gently. Cover and chill 2 hours.
Brush tuna steak with 1/2 teaspoon olive oil. Place on rack of a broiler pan coated with cooking spray. Broil 5 1/2 inches from heat (with electric oven door partially opened) 3 to 4 minutes on each side or until fish flakes easily when tested with a fork. Flake fish into pieces.
Place spinach on a serving plate. Arrange green bean mixture, tuna, tomato, and olives evenly over spinach. Drizzle remaining vinegar mixture evenly over salad.