Place the frozen fruit in the bottom of the slow cooker.
Mix together the pomegranate juice, cornstarch, and vanilla; pour over the frozen fruit.
In a medium bowl, mix together the oats, flour, brown sugar, baking powder, and salt. Use your fingers to mix the butter into the dry ingredients, until the butter is broken into small pieces. Sprinkle the mixture over the fruit.
Place 3-4 layers of paper towels over the top of the slow cooker and place the lid on top (this will catch any steam created by the slow cooker during cooking and prevent your topping from getting soggy).
Cook on low for 4-5 hours, or until the juices are thick and bubbly. Make sure not to lift the lid on the slow cooker for the first 3 hours of cooking.
Serve warm with your favorite vanilla ice cream.
Notes
I adapted this recipe to fit my 3 ½-quart slow cooker, but it can easily be doubled to fit a 6-quart.
Feel free to replace mango slices with peach slices.