Begin by washing the radishes thoroughly under cool running water. Using a mandoline slicer, thinly slice the radishes into even rounds; aim for about 1/8-inch thickness.

Next, wash the English cucumber, then slice it into thin rounds or quarters. Add the cucumber pieces to the bowl with the radishes.

Chop the spring onions thinly on a diagonal, and finely chop the dill. Combine with the radishes and cucumbers; toss lightly.

Crumble the feta cheese and gently add it to the vegetable mixture, ensuring it is evenly distributed.

In a separate bowl, combine the olive oil, apple cider vinegar, salt, and pepper. Whisk until emulsified.

Pour the dressing over the salad mix, and toss gently until all ingredients are coated.

Cover the bowl and refrigerate for about 30 minutes to allow flavors to meld.