1. Cook the Pasta: Bring a large pot of salted water to a rolling boil. Add the rotini pasta and cook according to package directions until al dente. This usually takes about 10-12 minutes.

2. Drain and Cool the Pasta: Once cooked, drain the pasta thoroughly in a colander. Rinse it under cold water to stop the cooking process and cool it down quickly. Let it drain completely.

3. Prepare the Chicken and Vegetables: While the pasta cooks and cools, prepare your other ingredients. Shred or dice your cooked chicken into bite-sized pieces. Halve the cherry tomatoes and finely dice the red onion.

4. Combine Ingredients in a Large Bowl: In a large mixing bowl, combine the cooled pasta, cooked chicken, halved cherry tomatoes, and diced red onion.

5. Add the Dressing: Pour the basil pesto, olive oil, and lemon juice over the pasta mixture. Add the grated Parmesan cheese.

6. Season and Toss: Season generously with salt and black pepper to taste. Gently toss all the ingredients together until everything is evenly coated with the pesto dressing. Make sure the chicken and vegetables are well distributed throughout the pasta.

7. Chill Before Serving (Optional but Recommended): For the best flavor, cover the bowl and refrigerate the pasta salad for at least 30 minutes to allow the flavors to meld. This also ensures it’s perfectly chilled for serving.

8. Garnish and Serve: Before serving, give it another quick toss. If desired, garnish with fresh chopped basil leaves for an extra burst of freshness and color.