Place beans in a 6 quart stock pot; 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 bow, 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.