Step 1: In a large bowl, whisk together the balsamic vinegar, olive oil, oregano, Dijon mustard, then add a little salt and pepper to taste.
Step 2: Add the diced cucumber and thinly sliced onion. Let it marinate for at least 5 minutes.
Step 3: In a bowl with the dressing, add the shredded lettuce, chickpeas, halved grape tomatoes, and pitted olives. Toss to combine and sprinkle with the crumbled feta cheese.