Wash the cucumbers thoroughly and pat them dry. Using a sharp knife or mandoline, slice the cucumbers into 1/8-inch thick rounds.
Place the cucumber slices in a large mixing bowl. Add the sliced garlic and chiles to the bowl with the cucumbers.
In a small bowl, whisk together the rice vinegar, soy sauce, sesame oil, sugar, salt, and black pepper until the sugar dissolves completely.
Pour the dressing over the cucumber mixture. Add the sliced green onions and toss everything together until well combined.
Let the salad sit at room temperature for 10-15 minutes to allow the flavors to meld. Toss once more before serving.
Transfer to a serving dish and sprinkle with toasted sesame seeds if using. Serve immediately or refrigerate until ready to serve.
