Boil the potatoes in salted water until fork-tender, about 10-12 minutes. Drain and let them cool slightly.
In a small bowl, whisk together olive oil, red wine vinegar, lemon juice, Dijon mustard, salt, and pepper.
In a large bowl, combine the cooled potatoes, cherry tomatoes, cucumber, olives, red onion, and feta (if using).
Pour the dressing over the salad and toss gently to coat everything evenly.
Top with fresh parsley before serving. Serve warm, chilled, or at room temperature—whatever makes you happy.