Wash and dry the cucumbers. Using a sharp knife or mandoline, slice them into thin rounds about 1/8-inch thick and place them in a large mixing bowl.
Add the thinly sliced red onion and chopped cilantro to the bowl with the cucumbers.
In a small bowl, whisk together the fresh lime juice, lime zest, olive oil, and salt until well combined.
Pour the dressing over the cucumber mixture and toss gently until everything is evenly coated.
Add the minced jalapeño or serrano pepper and toss again to distribute the heat evenly.
Let the salad sit at room temperature for 5-10 minutes to allow the flavors to meld.
Taste and adjust seasoning with more salt or lime juice if needed.
If using, sprinkle crumbled cotija cheese over the top just before serving.
Serve immediately as a refreshing side dish.