Slice the cucumbers thinly, about 1/8-inch thick. A mandoline works best, but a sharp knife is fine.
Toss the cucumber slices with 1 teaspoon salt in a colander. Let sit for 10 minutes to draw out excess water.
In a small bowl, whisk together rice vinegar, sugar, soy sauce, and sesame oil until the sugar dissolves.
After 10 minutes, gently squeeze the cucumber slices to remove excess liquid. Do not rinse.
Transfer the cucumbers to a serving bowl. Pour the dressing over and toss to coat evenly.
Sprinkle with toasted sesame seeds and red pepper flakes if using. Serve immediately or refrigerate for 30 minutes for more flavor.