When filled with sample-size portions of food, kitschy disposable cups and the salad inside seem so cool.
Whisk together mustard and vinegar in a small bowl. Add oil in a slow, steady stream, whisking constantly until emulsified. Season with salt and pepper.
Toss vegetables and olives in a medium bowl, and toss with vinaigrette. Season with salt and pepper. Divide salad among 12 three-ounce paper nut cups, top each with some feta, and serve immediately.
If you can't find baby cucumbers, cut English cucumbers small enough to fit in the cups.