Bring a large pot of well-salted water to a boil, then add the rotini pasta. Cook according to package instructions until al dente, about the time shown on the box, then drain.

Rinse the drained rotini briefly under cold water to stop cooking, then transfer it to a large mixing bowl. Spread it out slightly so steam escapes and it can absorb the dressing.

In a small bowl, whisk together reserved oil from sun-dried tomatoes, olive oil, red wine vinegar, Dijon mustard, garlic clove, dried oregano, red pepper flakes, salt, and black pepper until emulsified. Whisk for about 30 seconds until it looks glossy and uniform.

Pour about two-thirds of the dressing over the warm rotini and toss to coat. The pasta should look lightly glossy, and you should see the twists darken as it absorbs.

Add sun-dried tomatoes in oil, fresh mozzarella balls (ciliegine), Kalamata olives, medium red onion, and roasted red peppers, then toss gently to combine. Use gentle motions so the mozzarella stays in soft, visible pieces.

Fold in baby arugula or fresh basil leaves and Parmesan shavings. Toss just until the greens wilt slightly and the Parmesan disperses.

Taste the pasta salad and add the remaining dressing as needed, adjusting salt and black pepper. Add small splashes and toss between adjustments until balanced.

Cover and refrigerate for at least 20 minutes before serving to let the flavors meld. Chill until cold, then toss once more before serving for even coating.