Pat chicken dry and brush with olive oil. Season evenly with salt and pepper. Heat grill pan over medium-high heat until smoking. Cook chicken 5-6 minutes per side until internal temperature reaches 165°F and juices run clear. Transfer to a plate, tent with foil, and rest 10 minutes before dicing into bite-sized pieces.

Bring a large pot of salted water to a rolling boil. Add rotini and cook 1 minute less than package directions for al dente texture, usually 9-10 minutes. Drain and rinse under cold water until completely cool. Shake off excess water and transfer to a large mixing bowl.

While pasta cooks, dice cucumber into 1/2-inch pieces, halve the cherry tomatoes, and slice red onion as thin as possible. Add all vegetables to the bowl with cooled pasta along with chopped parsley.

In a small bowl, whisk Dijon mustard and honey until smooth. Add mayonnaise, apple cider vinegar, garlic powder, and black pepper. Whisk until fully emulsified and creamy. Taste and adjust - add more honey for sweetness or vinegar for tang.

Add diced chicken to the pasta and vegetables. Pour dressing over top and fold gently until everything is evenly coated. The pasta will absorb some dressing as it sits. Taste and season with additional salt and pepper if needed. Serve immediately or chill up to 2 hours.