Wash the cucumbers thoroughly. Peel if desired, then slice them in half lengthwise. Remove the seeds using a spoon and cut into half-moon slices.
In a large mixing bowl, combine the sliced cucumbers, red onion, and chopped cilantro.
In a separate small bowl, whisk together the fish sauce, lime juice, sugar, and minced garlic until the sugar is dissolved.
Pour the dressing over the cucumber mixture. Toss gently to combine all ingredients well.
If using, add the sliced red chili and toss again for a bit of heat.
Allow the salad to sit for about 5 minutes to let the flavors meld.
Before serving, sprinkle the chopped roasted peanuts on top for added crunch.
Taste and adjust the seasoning, adding more sugar or chili if desired to suit your preferences.
Serve chilled or at room temperature as a refreshing side dish.