Slice the cucumbers: Thinly slice the cucumbers using a sharp knife or mandoline. If using English cucumbers, there’s no need to peel them.
Make the dressing: In a small bowl, whisk together the rice vinegar, lime juice, fish sauce, sugar, sesame oil, garlic, and ginger until the sugar dissolves.
Combine the salad: In a large bowl, toss together the cucumbers and red onion. Pour the dressing over the vegetables and gently toss to coat.
Let the flavors develop (optional): For the best flavor, let the salad sit for about 10–15 minutes so the cucumbers lightly absorb the dressing.
Finish and serve: Just before serving, add the cilantro, mint, chili, and peanuts. Toss lightly.
Serve chilled or at cool room temperature.