Coat the inside of your slow cooker with cooking spray. Pour the pie filling or fresh strawberries into the slow cooker.
In a bowl, mix together the cake mix and the melted butter; it will make a crumbly mixture. Sprinkle over the strawberries in the slow cooker. Sprinkle the walnuts, if you are using them.
Cover and cook on low for 2 to 3 hours. Cool for about 10 minutes, then scoop into serving dishes and top with the Cool Whip.