Rinse the arugula, pat dry, and remove any hard stems. Rinse and thinly slice the radishes. Season the radishes with salt. Trim and thinly slice the scallion. Crumble the feta cheese into small pieces. Drain the capers. Mix all the prepared ingredients in a large bowl.
Add all the ingredients for the dressing in a bowl. Mix until thoroughly combined, then season to taste with salt, pepper, and vinegar. Toss with salad.
Place the salad into small salad bowls, and serve.