Slice cucumbers into thin rounds (about 1/8-inch thick). If using regular cucumbers, peel, halve lengthwise, scoop out seeds, then slice.
Thinly slice the red onion into half-moons.
In a large bowl, toss cucumbers and onion with 1 teaspoon salt. Let sit for 15 minutes to draw out excess water.
Meanwhile, finely chop the dill, parsley, and mint.
In a small bowl, whisk together apple cider vinegar, olive oil, honey, remaining salt, pepper, and red pepper flakes if using.
After 15 minutes, drain the cucumbers and onion (do not rinse). Pat dry with a clean towel.
Add herbs and dressing to the bowl. Toss well to coat.
Cover and refrigerate for at least 30 minutes (or up to 4 hours) to let flavors meld.
Before serving, stir and taste—adjust salt or vinegar if needed. Serve chilled.
