Add the strawberries and rhubarb into a large mixing bowl. I removed any strings that were on the rhubarb.
In a small bowl, add in the granulated sugar and cornstarch. Stir to combine.
Pour the sugar/cornstarch over top of the berries and rhubarb and mix to coat.
Spread the strawberries and rhubarb out into a 9x13 pan and set aside.
In a large mixing bowl, add in the butter and cake mix. Mix until combined.
Add in the egg and mix until incorporated.
Take small pieces of the cake mix and flatten it out and place it on top of the strawberries and rhubarb. Continue to do that to cover the entire pan. It's too thick to spread with a spoon, so this is the best way.
Place into the preheated oven. Bake for 45 minutes until the edges are golden brown and the juice is bubbling up the sides.
Scoop into a bowl and serve with a scoop of ice cream!