Combine 4½ cups flour, 2¼ cups brown sugar, 1½ tsp salt, 2 tsp cinnamon and a generous ¼ tsp nutmeg. Cut in 1½ cups butter. Measure out 3 cups of the mixture and add ½ cup chopped walnuts. In the rest of the mixture, add 1½ tsp baking soda. When mixed, stir in 1½ cups sour cream and 3 beaten eggs. Mix until dry ingredients are moistened. Pour into greased 9×13-inch pan. Sprinkle topping mixture over batter and press down gently. Bake at 350 F for 60 to 65 minutes.