Cook the tri-color rotini pasta according to package directions until tender. Drain and rinse with cold water to stop cooking and remove excess starch (aim for cool to the touch).

Spread the rinsed pasta on a sheet pan in a single layer. Refrigerate briefly while you prep the remaining ingredients to keep the pasta from warming the salad.

Add the pasta, halved pepperoni, cubed mozzarella, halved cherry tomatoes, diced green bell pepper, sliced black olives, and diced red onion to a large bowl. Toss gently to distribute everything evenly and create a tri-color mix.

Pour in the Italian dressing, add the grated Parmesan, and sprinkle with Italian seasoning. Toss until the pasta looks glossy and evenly coated, with no dry spots.

Season with salt and pepper to taste. Toss again briefly so the seasoning is distributed throughout.

Refrigerate for at least 3 hours or overnight, tossing occasionally. The pasta salad should look saucier as the flavors soak in.

Toss again before serving and add more Italian dressing if needed. Serve cold, with the pasta, pepperoni, and vegetables holding their shape.