Chop walnuts coarsely and briefly roast in a coated pan. Clean the mushrooms and radishes and cut into slices. Stir vinegar with mustard, honey and olive oil. Season with salt, pepper and orange zest. Add mushrooms and radishes and mix well. Cut prunes into halves or quarters. Wash arugula and shake dry. Add prunes, walnuts and arugula with mushrooms and mix shortly. Serve on top of salad leaves.