Cook rotini pasta according to package directions in a large pot of salted boiling water until al dente. Drain well and rinse with cold water to stop cooking and cool the pasta. Set aside.

While pasta cooks, bring a small pot of salted water to a boil. Add the broccoli florets and blanch for 2-3 minutes, or until bright green and crisp-tender. Immediately drain and plunge into an ice bath for 1-2 minutes to stop cooking and retain color. Drain thoroughly and pat dry.

In a large mixing bowl, combine the cooled pasta, blanched broccoli, diced red onion, halved cherry tomatoes, cubed cheddar cheese, and sunflower seeds (if using).

In a separate small bowl, whisk together the mayonnaise, apple cider vinegar, granulated sugar, Dijon mustard, garlic powder, salt, and black pepper until smooth and well combined. Taste and adjust seasoning as needed.

Pour the creamy dressing over the pasta and vegetable mixture. Toss gently until all ingredients are evenly coated.

Cover the bowl and refrigerate for at least 1-2 hours to allow the flavors to meld. Stir again before serving.