Mix sliced strawberries and rhubarb together in a bowl, stir in brown sugar so everything is coated, add more is required. Set aside for at least 1 hour, longer depending on desired sweetness.
Mix up cake batter as per instructions.
Spray or grease bottom of a 9 x13″ cake pan.
Spread 2/3 strawberry/rhubarb mix on bottom of pan, and then pour cake mix over. Add the remaining berry mix on top of the cake batter.
Cook cake as directed
Once out of the oven, let cool for 5 minutes. Run knife along edge of pan to ensure it is not sticking on the edges. Place a cookie sheet on top of the cake and flip upside down. Leave this to cool. Note: You may have to loosen the cake for total drop down.
Serve with whipped cream or ice cream of your choice!