Thinly slice the cucumbers using a mandoline or sharp knife.
Place cucumber slices in a colander, sprinkle with 1/4 teaspoon of the salt, and let sit for 10 minutes to draw out moisture.
While cucumbers are draining, whisk together white wine vinegar, olive oil, sugar, remaining salt, and black pepper in a small bowl until sugar dissolves.
Pat cucumber slices dry with paper towels to remove excess moisture.
In a large mixing bowl, combine cucumbers, red onion, and fresh dill.
Pour dressing over cucumber mixture and toss gently until evenly coated.
Let salad rest at room temperature for 5-10 minutes before serving to allow flavors to meld.
Garnish with extra dill sprigs if desired and serve immediately.
