Begin by bringing a large pot of salted water to a rolling boil over medium-high heat. Add in the tri-color rotini pasta and cook according to the package instructions until al dente, usually around 8–10 minutes. Stir occasionally to prevent sticking, and once cooked, drain the pasta in a colander, letting it sit for a few moments to remove excess water.

Rinse the pasta under cold water to stop the cooking process and cool it down rapidly; this should take about 2–3 minutes. Once it's cool to the touch, transfer the pasta to a large mixing bowl.

Gently fold in the halved cherry tomatoes, sliced pepperoni, diced red onion, and green bell pepper into the cooled pasta. Follow this by adding the cubed mozzarella cheese, grated parmesan, and sliced olives.

In a separate mixing bowl, whisk together the olive oil, red wine vinegar, Italian seasoning, garlic powder, salt, pepper, and red pepper flakes until well combined and emulsified.

Pour the homemade vinaigrette over the pasta and vegetables. Gently toss until all the ingredients are evenly coated.

Cover the bowl with plastic wrap or a lid and refrigerate the pasta salad for at least 1 hour.