1. Bring large pot of water to a boil. Add green beans; boil until crisp-tender, about 6 minutes. Remove the green beans with a slotted spoon to a colander; immediately rinse with cold running water to stop the cooking. Lightly salt the boiling water. Add the pasta to the boiling water; cook pasta following package directions. Drain in a colander, rinsing with cold water; let the pasta cool.
2. In a large bowl, combine green beans, the cooled pasta, cucumber, red onion, tomatoes, tuna and olives until well mixed.Dressing:
3. In small bowl, whisk together the mustard, lemon juice, vinegar, sugar, salt, oregano and black pepper. While whisking, add the oil in a thin stream; continue to whisk the dressing until smooth and the oil is thoroughly incorporated.To serve:
4. Pour dressing over the top of the salad; gently toss the salad with 2 large spoons to evenly coat all the ingredients with the dressing. Serve. Makes 12 servings.