How to Make It
Tightly twist a 3-foot length of aluminum foil to create a rack for slow cooker. Shape foil into a zigzag across bottom of a 5- or 6-quart round or oval slow cooker, and add 1 cup water. Place lid on slow cooker, and set on LOW.
Stir together walnuts, brown sugar, and 3 tablespoons of the flour in a small bowl. Stir in 2 tablespoons melted butter. Butter and flour an 8-cup Bundt pan. Sprinkle brown sugar mixture in an even layer in prepared pan; set aside.
Whisk baking powder, baking soda, cinnamon, salt, allspice, and remaining 2¾ cups flour in a medium bowl.
Beat granulated sugar and remaining 1/2 cup butter in a medium bowl with an electric mixer on medium until smooth, about 2 minutes. Add egg, and beat on low until incorporated. Stir in vanilla.
Add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat on low speed just until blended after each addition. Fold in chopped apple. Transfer batter to prepared pan.
Place pan in slow cooker, and press into foil rack to ensure pan is level. Drape a clean kitchen towel over slow cooker, and place lid over towel. Pull towel taut under lid. Cook on LOW until a wooden pick inserted in center comes out clean, about 3 hours. Cool cake in pan on a wire rack until just cool enough to handle, about 15 minutes. Invert cake onto a wire rack, and cool completely, about 30 minutes.