Spray 9x13 pan with cooking spray. Preheat oven to 325 degrees F. Prepare cake mix as directed on box. Mix Jell-O as directed with HOT water only (omit the cold water step)....then add in the frozen strawberries.
Cover the bottom of the baking pan with the bag of marshmallows, then pour cake mixture over the marshmallows, then pour the Jell-O/strawberry mixture over cake mix.
Bake 35-40 minutes, can check with toothpick for doneness. After removing from oven, let cool on countertop for 20-30 minutes, then refrigerate until cooled completely. Invert cake onto serving platter.