Finely chop the red onion and let it sit for 5 minutes to mellow.
Place the cucumbers on a clean kitchen towel and gently pat them dry.
Using the flat side of a heavy chef’s knife or a mallet, gently smash the cucumbers to create a texture that releases some juice while keeping them mostly intact.
In a large bowl, combine the smashed cucumbers, chickpeas, and red onion.
In a small bowl, whisk together the olive oil, lemon juice, minced garlic, oregano, salt, and pepper to make the dressing.
Pour the dressing over the cucumber and chickpea mixture, gently tossing to coat.
Add the chopped olives, crumbled feta, and fresh parsley. Fold in the baby spinach or arugula just before serving.
Adjust salt and pepper to taste. Serve immediately or refrigerate for a few hours to let the flavors meld.