In a large bowl, stir together flour, 1/2 cup brown sugar, white sugar, 1/2 teaspoon cinnamon, baking powder and baking soda.
Beat in buttermilk, butter, and egg. Spread into a greased 9 inch square baking dish. Cover with plastic wrap and refrigerate overnight.
Remove plastic wrap and preheat oven to 350 degrees. In a small bowl, stir together remaining 1/2 cup brown sugar, 1/2 teaspoon cinnamon, nutmeg, and chopped walnuts. Sprinkle evenly over top of batter.