Combine 4–1/2 cups flour, 2–1/4 cups brown sugar, 1–1/2 tsp salt, 2 tsp cinnamon and a generous 1/4 tsp nutmeg. Cut in 1–1/2 cups butter. Measure out 3 cups of the mixture and add 1/2 cup chopped walnuts. In the rest of the mixture, add 1–1/2 tsp baking soda. When mixed, stir in 1–1/2 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.