1. In a mixing bowl, stir together onion, red or green pepper, rice and beans.
2. In a jar with a tight fitting lid, add vinegar, dry mustard, garlic, salt, pepper and vegetable oil. Shake until dressing is evenly mixed.
3. Pour dressing over bean mixture and stir to mix evenly. Chill for at least one hour. Serve cold as a side dish or main dish.