Heat oven to 350° F. Grease one 8- or 10-cup Bundt pan. In a small bowl, combine half the sugar, the melted butter, and 2 1/2 cups of the walnuts to make the topping. Spread the mixture evenly in the pan, covering the bottom evenly.
To make the filling, cream the remaining sugar and the softened butter in a medium bowl. Stir in the flour and the remaining walnuts. Set the mixture aside.
In a large mixing bowl, combine the cake mix, eggs, and oil, along with the amount of water called for by the package directions. Beat until well combined. Pour half the batter into the pan, covering the topping. Spoon the filling mixture evenly over the batter, then pour the remaining batter on top, covering the topping. Place in oven and bake until a knife inserted into the center comes out clean, about 45 minutes. Let cool 10 minutes, then unmold bottom-side up onto a cake plate. Serve warm or at room temperature.