In a large bowl, combine the sliced cucumbers and red onion. Sprinkle with salt and let sit for 10 minutes to draw out moisture. Pat dry with paper towels.
While the cucumbers are salting, whisk together the rice vinegar, soy sauce, sesame oil, honey, minced garlic, and red pepper flakes (if using) in a small bowl.
Pour the dressing over the cucumbers and red onion. Toss gently to coat.
Stir in the chopped cilantro and roasted peanuts (if using).
Serve immediately or chill for at least 30 minutes for flavors to meld.
Notes
This salad is best served fresh, but leftovers can be stored in an airtight container in the refrigerator for up to 2 days.