Combine mayonnaise, sour cream, dill, vinegar, sugar, salt, and pepper in a small bowl and mix until well blended. Set aside.
Place the minced onion slices in cold water for a few minutes to reduce sharpness, then drain.
Cook the penne pasta in boiling water until al dente according to package instructions. Drain and rinse with cold water to cool and stop cooking.
Cut the cucumber in half lengthwise, scoop out seeds with a spoon, then slice into thin pieces.
In a large bowl, combine cooled pasta, drained onion slices, and cucumber slices.
Pour the prepared dressing over the salad ingredients and toss gently until fully coated.
Refrigerate the salad for at least 40 minutes before serving to let flavors meld.