Using a mandoline (or a knife), thinly slice 6 cucumbers ¼ inch thick.
Transfer the cucumber slices to a fine-mesh strainer set over a large bowl; sprinkle with 2 teaspoons sugar and ½ teaspoon salt. Toss to combine. Let stand at room temperature for 30 minutes.
Discard any liquid in the bowl and wipe it dry.
Transfer the cucumbers to the bowl and toss with 1 cup onion, ¼ cup vinegar, 2 tablespoons dill and ¼ teaspoon pepper.
Let stand at room temperature for 15 minutes.
Serve immediately or refrigerate for up to 3 days.
