In a small bowl, whisk together the extra virgin olive oil, balsamic vinegar, Dijon mustard, garlic powder, and salt until well combined and emulsified. Set aside.

In a large mixing bowl, combine the mixed leafy greens, diced cucumber, halved cherry tomatoes, sliced red onion, Kalamata olives, and crumbled feta. Toss gently.

Just before serving, pour the vinaigrette over the salad and toss to coat evenly. Serve immediately.