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

In a large salad bowl, add the mixed leafy greens, cucumber, cherry tomatoes, red onion, Kalamata olives, and feta cheese. Toss gently to combine.

Drizzle the vinaigrette over the salad and toss gently with salad tongs to coat evenly.

Transfer the salad to a flat serving platter and serve immediately.