Wash cucumbers thoroughly and slice them into thin, uniform rounds. Place sliced cucumbers in a large colander and sprinkle generously with salt. Toss gently and let stand for 30 minutes.

After 30 minutes, rinse cucumbers thoroughly under cold water to remove excess salt. Gently squeeze cucumber slices to remove as much water as possible.

While cucumbers are draining, slice the onion into thin half-moons. If using regular yellow onion, soak sliced onions in ice water for 10 minutes, then drain and pat dry.

In a large mixing bowl, whisk together sour cream, mayonnaise, vinegar, sugar, salt, and black pepper until smooth and well combined.

Add the drained cucumbers and sliced onions to the dressing. Sprinkle fresh dill and chives over the mixture. Gently fold all ingredients together until cucumbers are evenly coated.

Cover the bowl with plastic wrap and refrigerate for at least 2 hours before serving. For best results, chill for 4 hours.