Make dressing: Whisk together vinegar, garlic, oregano, salt, pepper and sugar. Gradually drizzle in oil, whisking constantly, until dressing is well mixed.

Make salad: In a large bowl, combine lettuce, mozzarella, roasted red and pepperoncini peppers, olives, salami, basil and onion; toss gently to combine. Just before serving, pour dressing over salad and toss to coat.