Prepare the vinaigrette by whisking together the olive oil, balsamic vinegar, Dijon mustard, garlic powder, and salt in a small bowl until emulsified.

Combine salad ingredients in a large mixing bowl, starting with the leafy greens, cucumber, cherry tomatoes, red onion, Kalamata olives, and feta cheese.

Toss the salad with the prepared vinaigrette until everything is evenly coated.

Taste and adjust the dressing with more salt or olive oil if necessary. Serve immediately.