Place sliced cucumbers and radishes in a large bowl. Sprinkle with salt and toss well. Let sit for 10 minutes.
After 10 minutes, transfer the vegetables to a colander and rinse briefly with cold water. Pat very dry with paper towels or a clean kitchen towel.
In a small bowl, whisk together lemon juice, lemon zest, olive oil, honey, and a few grinds of black pepper.
In a large serving bowl, combine the dried cucumbers and radishes with the dressing and fresh dill. Add red onion if using. Toss gently to coat.
Let the salad rest for 5 minutes at room temperature to allow flavors to meld. Serve chilled or at room temperature.
