01 - Preheat oven to 350°F. Lightly grease a 9×13-inch baking dish with butter or cooking spray.
02 - Pour the lemon pie filling into the bottom of the prepared baking dish, spreading it into an even layer with a spatula.
03 - Distribute the mandarin orange segments evenly over the lemon filling. Sprinkle the fresh lemon zest across the top.
04 - Sprinkle the dry yellow cake mix evenly over the entire fruit layer. Do not stir or mix the layers.
05 - Drizzle the melted butter and whole milk evenly over the cake mix, ensuring coverage of as much dry mix as possible. If using, sprinkle the granulated sugar over the top.
06 - Bake for 35–40 minutes until the top is golden brown and the edges are bubbling.
07 - Allow the dump cake to cool for at least 15 minutes before serving. Serve warm with whipped cream or vanilla ice cream if desired.