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

Thinly slice the sweet white onion and soak it in cold water for about 10 minutes to mellow its sharpness.

Bring salted water to a boil, add pasta, and cook according to package instructions until al dente. Drain and rinse under cold water.

Cut the cucumber in half lengthwise, scoop out seeds, and slice into thin half-moons.

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

Cover and refrigerate for at least 40 minutes before serving.