Place the washed cucumber on a firm, flat, clean surface. Whack it firmly but gently with the flat side of a heavy cleaver or the bottom of an 8-inch skillet. Repeat all down its length until it is completely smashed. Chop the cucumber crosswise into 1-inch pieces and transfer to a colander set in a bowl. Toss with salt and refrigerate for 10 minutes. Discard the extracted liquid and transfer the cucumbers to the bowl.
Add garlic, vinegar, and sesame oil (if using) to the bowl and toss to combine. This cucumber salad is best served absolutely fresh; It will get wet and slimy as it rests in the fridge.
