In a small mixing bowl, combine sour cream, mayonnaise, sugar, salt, dill, and vinegar. Whisk until smooth and creamy, then set aside.

Soak sliced onion in cold water for 10 minutes, then drain and set aside.

In a large pot, bring salted water to a boil, add pasta, and cook according to package instructions (8–10 minutes). Drain and rinse under cold water.

Cut cucumber in half lengthwise, remove seeds, and slice into thin half-moons.

In a large bowl, combine cooled pasta, soaked onions, and sliced cucumbers. Pour dressing over the top and gently toss until evenly coated.

Cover and refrigerate for at least 40 minutes before serving to allow flavors to meld.