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

Thinly slice the sweet white onion and soak in cold water for about 10 minutes. Drain well and set aside.

Bring a large pot of salted water to a boil. Add pasta and cook until al dente, about 8 to 10 minutes. Drain and rinse under cold water.

Slice the cucumber in half lengthwise, scoop out the seeds, and cut into thin half-moons.

In a large bowl, combine cooled pasta, drained onions, and sliced cucumbers. Add dressing and toss gently until coated.

Cover and chill in the refrigerator for at least 40 minutes before serving to allow flavors to meld.