1. Arrange potato and green beans on one side of a steamer basket over boiling water in a Dutch oven. Place tuna on opposite side of basket. Cover and steam 8 to 10 minutes or until fish flakes easily when tested with a fork. Set tuna aside to cool. Plunge steamed potato and green beans into ice water to cool.
2. While tuna and vegetables steam, combine vinegar, lemon juice, and mustard in a jar; cover tightly, and shake vigorously.
3. Place lettuce on a serving platter. Drain potato and beans; arrange over lettuce. Flake tuna, and place on salad; add tomato wedges. Drizzle with vinegar mixture. Sprinkle with freshly ground black pepper.