1. Bring a large pot of water a rolling boil. Add in the tortellini. Do NOT cook tortellini according to package instructions. Instead, boil for exactly 90 seconds. Then drain and rinse with cold water.
2. Place drained tortellini in a large bowl. Add in the pesto, tomatoes, mozzarella, onion, and basil. Toss to coat. Season with salt and pepper.
3. So easy, right?! Serve at once, or refrigerate until needed.