Prepare the following, adding each to the same medium bowl as you complete it: Using a mandoline or a knife, thinly slice 1 pound English or seedless cucumbers crosswise (4 to 5 cups). Halve and thinly slice 1 small red onion (about ½ cup). Sprinkle with 1 ¼ teaspoons kosher salt and toss to combine. Let sit for 20 minutes.
Pour off any liquid from the cucumber mixture. Add 3 tablespoons vinegar of choice and 2 teaspoons granulated sugar, and toss until the sugar is dissolved. Taste and season with black pepper as needed. Add 1 tablespoon coarsely chopped fresh dill if desired and toss to combine.
