First, segment your oranges. Cut off the very top and bottom of the orange (the orange will then sit flat). Remove the skin and bitter pith of the orange by hand or with a knife. Using a small knife, cut out each segment of the orange, leaving behind the membrane. Segment the orange over a bowl to collect the juice! Next, prepare your radishes. Wash them well, scrubbed if necessary, then coarsely grate. Combine the radishes, orange segments and juice, add a little salt and pepper, and toss. Refrigerate for a few hours. Sprinkle some finely diced walnuts, and serve!