01 - Preheat oven to 350°F. Lightly grease a 9×13-inch baking dish with butter or cooking spray.
02 - Spread lemon pie filling evenly across the bottom of the prepared baking dish.
03 - Distribute mandarin orange segments over the lemon filling. Sprinkle fresh lemon zest evenly across the fruit layer.
04 - Pour dry yellow cake mix over the fruit layer in an even layer. Do not stir or mix the ingredients.
05 - Drizzle melted butter and whole milk evenly over the cake mix, ensuring dry powder is thoroughly moistened. Sprinkle granulated sugar over the surface if desired.
06 - Bake for 35–40 minutes until the top is golden brown and edges are bubbling with fruit filling.
07 - Let the cake rest for at least 15 minutes before serving. Serve warm with whipped cream or vanilla ice cream if desired.