Prepare your crockpot. Spray the liner of your crockpot with nonstick spray.
Add the filling. In the basin of your crockpot, combine the fresh rhubarb and strawberry jelly. Stir well to coat all of the rhubarb with the jelly.
Make the crumble topping. In a large bowl, add the cake mix and the butter. Use a fork or clean hands to crumble the softened butter throughout the cake mix. Work the butter into the cake mix until it resembles a damp sand texture.
Top the filling with the crumble. Sprinkle this cake mixture over the rhubarb in the crockpot, making sure to cover all of the rhubarb and jelly.
Cover and cook. Cover and cook on high for 2 hours until the crust is golden and the filling is bubbly. If the topping isn’t golden or still looks raw and gooey, cook for another hour or until it's golden and firm.