1 Preheat the oven to 325ºF and line cookie sheets with parchment paper. In a food processor fitted with the metal blade, combine the walnuts, 1/2 cup of the sugar, the flour, cinnamon, cloves, allspice and salt. 2 Process until the walnuts are finely ground, then transfer to a large bowl. 3 In a medium bowl, beat the egg whites until soft peaks form. Add the lemon juice, then gradually add the remaining 1/2 cup of sugar as you continue beating until stiff peaks form. Scoop the beaten whites on top of the walnut mixture, then stir together until completely blended and you have a stic4 Though not essential, the dough will be easier to handle if chilled for a couple of hours before rolling it out. It will keep, refrigerated, for several days.) 5 Place about a 14-inch sheet of parchment paper on your work surface and sprinkle with 3 - 4 tablespoons of sugar. Put half the dough on the sugared parchment and sprinkle it with 2 more tablespoons of sugar. Cover with another sheet of parchment. Roll the dough between the parchment to a thickne6 Peel away the top sheet of parchment. Using a 2 - 2 1/2-inch star (or round) cutter, cut as many cookies as you can. With a spatula, transfer to the prepared baking sheets. Reroll the scraps and cut more cookies. (You can reuse the parchment, sprinkling it with sugar each time.) 7 Bake for 15 - 18 minutes, until the cookies look dry. Cool for 2 minutes on the pan, then transfer to racks to cool completely. 8 To prepare the icing, combine the powdered sugar and water in a bowl and stir briskly, using enough liquid to make a smooth,9 Brush the cookies with a thin coating of icing and, before it dries, decorate with a walnut half or sprinkle lightly with chopped walnuts. When the icing is dry and set, store the cookies in an airtight container.