Cook the rotini pasta according to package instructions. Drain and rinse under cold water to cool.
In a large bowl, combine the cooked pasta, shredded chicken, steamed broccoli, cherry tomatoes, and red onion.
In a separate bowl, whisk together the mayonnaise, Greek yogurt, Dijon mustard, lemon juice, garlic powder, salt, and pepper until smooth.
Pour the dressing over the pasta mixture and toss until everything is well coated.
If desired, stir in the shredded cheddar cheese.
Cover and refrigerate for at least 30 minutes to allow the flavors to meld. Serve chilled or at room temperature.