1. Combine farro, 1/2 teaspoon salt, and 4 cups water in a medium saucepan; bring to a boil. Cover and simmer 60 minutes or until farro is tender but still slightly chewy. Drain. Cool slightly.
2. While farro cooks, fill another medium saucepan two-thirds full with water; bring to a boil. Add green beans; cook 4 minutes or until crisp-tender. Drain and rinse with cold water; drain.
3. Combine oil, vinegar, juice, mustard, and garlic in a large bowl; stir in the remaining 1/4 teaspoon salt and pepper. Add farro, beans, tomatoes, and next 4 ingredients (through green onions); toss well to combine. Flake tuna into large chunks. Add tuna to salad; toss gently to combine.