In a bowl whisk together the flour, the baking soda, the salt, the cinnamon, the nutmeg, and the cloves. In another bowl with an electric mixer cream the butter with the brown sugar, beat in the egg, and beat in the molasses. Add the flour mixture, beating the mixture until it is just combined, and stir in the raisins and the walnuts. Spread the mixture in a buttered and floured 13- by 9-inch baking pan and bake it in the middle of a preheated 350°F. oven for 15 to 20 minutes, or until a tester comes clean. Let the mixture cool completely in the pan on a rack and cut it into 24 bars.