Bring a large pot of salted water to a boil. Cook the tri-color rotini according to the package directions until just tender, generally 8 to 10 minutes.

Drain the pasta thoroughly, then rinse it briefly under cold water to stop the cooking and cool the noodles. Shake off as much excess water as possible so the dressing does not become watery.

Place the cooled pasta in a large mixing bowl. Add the halved tomatoes, cucumber, red onion, sliced olives, mozzarella, and pepperoni if using.

In a small bowl, whisk together 3/4 cup Italian dressing, Parmesan, Italian seasoning, salt, and black pepper. Pour the mixture over the pasta and vegetables.

Toss gently until every rotini spiral and vegetable is evenly coated. Use a soft folding motion to keep the mozzarella intact and prevent the tomatoes from breaking apart.

Cover the bowl and refrigerate for at least 30 minutes, although 1 to 2 hours gives the flavors more time to mingle. Toss again before serving, adding the remaining dressing if the salad looks dry.

Garnish with chopped fresh parsley just before serving. Taste once more and adjust the salt, pepper, or dressing as needed.