Mix the brown sugar, cinnamon, and nutmeg together in a bowl. Add the apples and cranberries, tossing to coat evenly with the sugar mixture. Pour into a slow cooker. Top with butter pieces. Mix the oatmeal together with the water, apple juice, cranberry juice, and salt in a bowl, and pour over the apple mixture in the slow cooker. Do not stir. Cover, and cook on Low for 8 hours. Stir before serving. Spoon into bowls and top with candied walnuts, if desired.