Step 1Cut the onion in half lengthwise, then slice it crosswise into semi-circles. Place the onion in a bowl, cover it with cold water and add the vinegar. Let the onion sit for at least 10 minutes, preferably longer, while you prepare the remaining ingredients.
Step 2If the tomatoes are thick-skinned, peel them, then cut them into wedges, and cut the wedges in half if they're very large. Peel the cucumber, if you prefer, then cut it in half lengthwise, then slice it. Cut the bell pepper into 1-inch pieces or rings.
Step 3Drain the onion and place it in a serving bowl. Toss it with the tomatoes, cucumber, bell pepper, salt and pepper to taste, vinegar, oil, feta, mint and olives. Taste, adjust the salt and pepper, and serve.