We’ve done these before. The recipe, such as it is, is simple enough: Dump some fruit into a pan, scatter a handful of rolled oats over the fruit, dump a cake mix atop that, and then dump butter on top of the mix. Bake in a 350 oven till the top browns and the fruit bubbles up.
I went with a bag of blackberries mixed with some Splenda for sweetener and a bit of lemon juice and a teaspoon of cinnamon. Some recipes go with raw fruit but I opted for cooking mine down and adding some corn starch as a thickener. I was breaking in a new 8×8 pan so I used only a half box of white cake mix over the rolled oats, and used a grater to shred a stick of very cold butter right over that. Using a 9×13 cake pan you would use the entire box of mix and two sticks of butter.