Whisk together the lime juice and olive oil. Season with salt and pepper and set aside.
Thinly slice the radishes, cucumber and onion, ideally on a mandoline: you will end up with discs of radish and cucumber and half-moons of onion.
Put the vegetables in a shallow bowl and toss with the dressing. Cover with plastic wrap and set aside for 20 minutes.
Serve sprinkled with