In a small bowl, combine diced red onion, vinegar and sugar; set aside to "quick-pickle" the onions for a few minutes. In a medium bowl, mix the shredded cabbage and raisins together, then add the onions, making sure to leave the vinegar solution in the bowl to create the dressing. Add mayonnaise, salt and pepper to the "quick-pickle" vinegar solution and stir until well incorporated. Mix the dressing with the bowl of cabbage, raisins and onions, then let slaw chill in the fridge to help marry flavors. Fold in diced peaches just before serving.