Place beans in a large saucepan; cover with water. Bring to a boil. Cook, uncovered, for 8-10 minutes or until crisp-tender. Meanwhile, for dressing, whisk the vinegar, oil, mustard, salt and pepper in a small bowl. Drain beans; place in a large bowl. Add tomatoes and onion. Drizzle with dressing and toss to coat. Sprinkle with parmesan cheese.