Boil the halved potatoes in salted water for 12–15 minutes or until fork-tender. Drain and let cool slightly.
In a large bowl, combine potatoes, red onion, olives, cucumber, and parsley.
Whisk together olive oil and lemon juice. Season with salt and pepper.
Pour the dressing over the salad and toss gently to combine.
Top with crumbled feta if using and chill before serving.