Combine first 3 ingredients in a small bowl; add 1/4 teaspoon cinnamon, stirring to combine. Peel top third of each apple; place apples in an electric slow cooker. Spoon walnut mixture into cavity of each apple.
Combine the remaining 1/2 teaspoon cinnamon, 1 cup brown sugar, and apple cider in a small bowl, stirring to combine. Pour over apples. Cover with lid; cook on LOW 2 3/4 hours. Remove the apples with a slotted spoon. Spoon 1/4 cup cooking liquid over each serving.