Combine the walnuts, cranberries, brown sugar, oats, butter, lemon juice, cinnamon and 1/2 teaspoon salt in a small bowl and squeeze together until a wet and sandy mixture is formed.
Scoop the core out of each apple with a melon baller, taking care not to scoop completely through to the bottom or the sides. Stuff each apple with some of the cranberry-walnut mixture. Place the apples in the slow cooker insert and pour in the apple cider. Cover and cook on low until the apples are tender but still slightly firm, 3 to 4 hours depending on the size and type of apple. (If your apples are on the smaller size, check after 2 hours; they may cook faster.)
Serve each apple whole or cut in half in a bowl with a scoop of ice cream and a drizzle of maple syrup.
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