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Preheat oven to 350 degrees. Line an 8-inch round pie plate with parchment paper and then generously grease the parchment with butter or margarine. Fan out the slices apples around the bottom of the pan. Set aside.
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In the bowl of a standing mixer or in a large mixing bowl, beat buttermilk, sugar, oil, and egg until combined. Add in the flour, baking soda, nutmeg, salt, and chopped apples and stir until combined. Do not over mix. Pour into prepared pan and bake for 45 minutes or until a cake tester or toothpick inserted into center of cake comes out clean. While cake is baking, prepare the Walnut Topping.
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In a small bowl, combine the brown sugar, margarine or butter, and walnuts with a fork until the mixture resembles coarse crumbs. Set aside.
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When the cake is done, remove from oven and allow it to cool for 10 minutes. Run a knife along the edges of the pan to loosen the cake and carefully invert it onto a plate or cake stand. Gently remove the parchment paper and sprinkle the walnut topping on top of the cake. Serve warm.