Preheat your oven to 350°F. While it heats, prepare the fruit by cutting the rhubarb into 1-inch pieces and hulling and slicing the strawberries into 1/4-inch thick slices. This prep work ensures everything is ready when you need to layer the dump cake, preventing any unnecessary delays during assembly.

In a small bowl, whisk together the granulated sugar, cinnamon, and salt until well combined. Set aside 1/4 cup of this mixture in a separate small bowl—this will be your flavor layer for the strawberries. I like to reserve the spiced sugar before mixing it all together because it makes the layering process cleaner and ensures the strawberries get their own aromatic boost.

Spread the cut rhubarb evenly in a 9x13 inch baking dish. Sprinkle the larger portion of the spiced sugar mixture (about 3/4 cup) directly over the rhubarb, then layer the sliced strawberries on top. Sprinkle the reserved 1/4 cup of spiced sugar mixture over the strawberries, then drizzle the lemon juice over all the fruit. This layering technique allows the sugar to draw out the fruit juices during baking, creating a flavorful syrup.

Sprinkle the strawberry gelatin powder evenly over the fruit layer, followed by an even distribution of the yellow cake mix. Don't stir—let these dry ingredients sit on top of the fruit. The gelatin will rehydrate as the cake bakes, adding extra fruity flavor and moisture to the topping.

Drizzle the melted and cooled butter evenly over the dry cake mix topping, allowing it to soak into the cake mix gradually. Pour the water around the edges of the dish and over the top—the water will work with the butter and cake mix to create a cake-like texture as it bakes. Place the baking dish in the preheated 350°F oven and bake for 50-55 minutes, until the top is golden brown and a toothpick inserted into the cake portion comes out with just a few moist crumbs (not wet fruit).

Remove the dump cake from the oven and let it cool for at least 15-20 minutes before serving. I find that waiting a bit allows the cake and fruit layers to set slightly, making it easier to scoop and plate. The cake will continue to thicken as it cools. Serve warm or at room temperature with vanilla ice cream or whipped cream if desired.