In a small mixing bowl, combine sour cream, mayonnaise, sugar, salt, fresh dill, and vinegar. Whisk until smooth and creamy, about 2-3 minutes.

Thinly slice the sweet white onion and soak it in cold water for about 10 minutes, then drain.

Bring a large pot of salted water to a boil. Cook the medium pasta according to the package instructions, about 8-10 minutes, until al dente, then drain and rinse under cold water.

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

In a large mixing bowl, combine the cooled pasta, soaked onions, and sliced cucumbers. Gently fold to mix.

Pour the dressing over the pasta and vegetable mixture, toss gently to coat.

Cover the bowl with plastic wrap or transfer to an airtight container. Chill for at least 40 minutes before serving.