Bring a large pot of salted water to a rolling boil.
Add the tortellini and cook for 1½ minutes only (ignore package instructions).
Drain immediately and rinse under cold water to stop the cooking process.
In a large bowl, combine cooked and cooled tortellini, halved cherry tomatoes, red onion, mozzarella balls, and pesto or Italian dressing.
Toss everything together until evenly coated.
Serve immediately, or cover and refrigerate for up to 3 days.