Place the sliced cucumbers and red onion in a colander set over a bowl or in the sink. Sprinkle with 1 teaspoon salt and toss gently to coat. Let drain for 10 minutes to remove excess moisture.
While the vegetables drain, make the dressing: In a medium bowl, whisk together the mayonnaise, white vinegar, sugar, and black pepper until smooth and well combined.
Rinse the salted cucumbers and onion under cold water to remove excess salt. Pat them dry thoroughly with paper towels or a clean kitchen towel.
Transfer the dried vegetables to a large mixing bowl. Add the chopped fresh dill and pour the dressing over everything.
Gently toss everything together until the cucumbers and onion are evenly coated with the dressing.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld. Garnish with extra dill sprigs if desired before serving.
