Salt the sliced cucumber with ½ teaspoon of salt. Toss, rest for 10 minutes, shake well, and pat completely dry.
Soak sliced red onion in cold water for 5 minutes. Drain and pat dry.
Whisk all dressing ingredients together until combined. Taste boldly and adjust until bright, tangy, and well-seasoned.
Combine drained cucumber, tomatoes, red onion, parsley, and mint in a large bowl. Toss gently.
Pour dressing over the salad and toss to coat. Taste and adjust seasoning immediately.
Rest for 5 to 10 minutes before serving to allow flavors to meld and tomatoes to release their juices.
Scatter feta and olives on top if using. Finish with an extra drizzle of olive oil and torn mint leaves.
Serve immediately on a wide flat platter or bowl.