Place beans in a 6-qt. stockpot; add water to cover. Bring to a boil. Cook, covered, 8-10 minutes or until crisp-tender. Drain and immediately place in ice water. Drain and pat dry., In a small bowl, whisk oil, lemon juice, vinegar, salt, garlic powder, mustard and pepper. Drizzle over beans. Add onion; toss to coat. Refrigerate, covered, at least 1 hour. Just before serving, stir in tomatoes and cheese.