Heat the oven to 350°F. Butter and flour a 10-cup bundt pan and set aside.
For the streusel-Blend the brown sugar, ginger, cinnamon, flour, and butter with your fingers or a pastry blender until the mixture resembles large breadcrumbs. Mix in the walnuts and refrigerate until ready to use.
For the cake-Sift together the flour, ground ginger, nutmeg, baking powder, baking soda, and salt into a large bowl. Stir in the crystallized ginger and set aside. In a large mixing bowl, beat the butter with an electric mixer until light and fluffy.
Gradually add the sugars, continuing to beat until very light. Add the vanilla and then the eggs one at a time, beating each one in well before adding the next. Alternately stir in the flour and the buttermilk, until the batter is just mixed. Don't overbeat or your cake will be tough. Gently stir in the chopped apple.
Pour half the batter into the prepared pan. Sprinkle with a third of the streusel. Pour in the remaining batter and top with the remaining streusel. The streusel will sink a little during baking to make a swirl.
Bake until the top of the cake is firm (a skewer inserted in the center should come out clean), about 40 to 45 min. Cool for 15 min. in the pan on a wire rack. Invert the cake onto a serving platter, dust with confectioners' sugar, and serve slightly warm or at room temperature.