Toss the onion slices with
Arrange the cucumbers on a salad plate and top with the onions. Add the tomato slices and then the pepper slices, alternating colors if possible. Top with the feta and olives. Drizzle with the remaining vinaigrette.
To make Oregano Garlic Vinaigrette, rub the oregano with your hands to warm it and release its oils or toast it in a dry skillet for a minute or two. Whisk all the ingredients together.