Separate the leaves from the radishes, wash and dry them, then set aside. Finely grate the zest of half the lemon, then juice all of it. Tip the zest into a salad bowl and toss through the shallots. Leave to macerate while you prepare the rest of the salad.
Trim the fennel and slice it as finely as possible, then quarter the radishes. Just before serving, toss all the ingredients, including the radish leaves, with the lemon juice and olive oil. Season with salt and black pepper, then serve.