Wash the cucumbers thoroughly and pat them dry. Using a sharp knife or mandoline, slice the cucumbers into thin rounds, about 1/8-inch thick.
Place the cucumber slices in a colander set over a bowl. Sprinkle with 1 teaspoon of salt and toss gently to coat. Let them sit for 10 minutes - this draws out excess moisture and keeps them crisp.
While the cucumbers are resting, make the dressing. In a small bowl, whisk together the rice vinegar, sugar, and soy sauce until the sugar dissolves completely.
After 10 minutes, gently squeeze the cucumber slices to remove any remaining liquid. Transfer them to a clean mixing bowl.
Pour the dressing over the cucumbers and toss gently to coat every slice. Let the salad sit for another 5-10 minutes to allow the flavors to meld.
Just before serving, sprinkle the toasted sesame seeds over the salad. If using, add the sesame oil and red pepper flakes at this point.
Give everything one final gentle toss and serve immediately. The salad can also be refrigerated for up to 30 minutes if you prefer it extra chilled.