Cook the pasta: Boil salted water and cook the pasta until tender, then drain and rinse under cold water.
Prepare the chicken: Cook chicken until 165°F (74°C), let it rest, then chop it into bite-sized pieces.
Prepare vegetables: Chop the cucumber, tomatoes, and bell pepper into small pieces.
Make dressing: Whisk mayonnaise, sour cream, lemon juice, garlic powder, salt, and pepper until smooth.
Combine: Add pasta, chicken, vegetables, and cheese to a large bowl.
Dress the salad: Pour the dressing over the ingredients and gently toss until evenly coated.
Chill: Cover and refrigerate for at least 30 minutes.
Serve: Toss gently once more and serve chilled.