In a small bowl or measuring cup, whisk the apple cider vinegar, olive oil, honey, parsley, and pepper. Set aside.
Slice the cucumber into ¼-inch thick slices.
Once cut, place in a colander lined with paper towels. Layer the cucumber slices, sprinkling salt in between each layer. This draws out the water in the cucumbers so the salad does not become watery.
Allow to sit for 10 minutes. Afterward, pat the sliced cucumbers and place them in a large bowl.
Slice the red onion into thin slices. Place them into the bowl with the cucumbers.
Pour the dressing over the cucumbers and onions and toss to coat.