Thinly slice the cucumbers. Place the cucumber and onion slices in a colander and toss with the salt. Let sit in the sink or over a bowl for 30 to 40 minutes If you have time) to catch the liquid that will drip out.
Rinse the cucumbers and onion in very cold water. Then, using your hands (or better yet, a clean dish towel), squeeze the liquid from the vegetables, getting as much water out as possible.
Transfer to a bowl and add the vinegar, oil, dill, sugar, and pepper. Toss to combine. Serve immediately, or refrigerate for a day, knowing the salad will soften as it marinates.
Notes
You can use two seedless cucumbers if you can’t find the small ones. Slice the cucumbers in half lengthwise and scoop out the small amount of seeds using a teaspoon (even seedless cucumbers have seeds!). Cut the cucumber halves into thin half-moon slices.
In a perfect world, you let the cucumbers and onions macerate (soften) in some salt for a bit, which pulls out some of the liquid from the watery cucumbers and softens the flavor of the onions a bit.
To remove the excess water from the cucumbers and onions after salting, just pile the rinsed cucumbers and onions in a line down the middle of a clean dish towel. Roll it up, and twist and squeeze over a bowl or the sink. Or just.use your hands.
If you like a super-crispy salad, make this right before serving. But, if you like (as I do) a salad with some crunch but a slightly more pickle-y marinated quality, then make it a day ahead.
If you don’t have the time to salt and drain the cucumbers and onion, you will still have a lovely, clean-tasting salad. But without salting first, the salad will get watery more quickly, so if you do skip that step, it’s best to serve it within a day.