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

Slice the sweet white onion thinly and soak in cold water for 10 minutes. Drain and set aside.

Boil salted water, add pasta, and cook al dente for about 8-10 minutes. Drain and rinse under cold water.

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

In a large bowl, combine pasta, onions, and cucumber. Pour dressing over and toss gently until coated.

Cover and refrigerate for at least 40 minutes before serving.