In a mixing bowl, combine yogurt or sour cream with mayonnaise, stirring until fully blended and creamy. Add salt and pepper to taste.

Slice the red onion thinly and place it in a bowl of ice-cold water for 10-15 minutes. Drain and set aside.

Bring a pot of salted water to a boil, then add pasta. Cook until al dente, about 8-10 minutes. Drain and rinse under cold water.

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

In a large mixing bowl, combine cooled pasta, drained onions, and cucumber slices. Toss gently.

Pour dressing over the salad and fold everything together gently until well coated.

Cover the bowl and refrigerate for at least 40 minutes before serving.