01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish thoroughly with butter or cooking spray.
02 - Spread lemon pie filling evenly across the bottom of the prepared dish using a spatula.
03 - Distribute mandarin orange segments over the lemon layer. Sprinkle fresh lemon zest evenly across the fruit.
04 - Pour dry yellow cake mix over the fruit mixture in an even layer. Do not stir or mix the layers.
05 - Drizzle melted butter and whole milk evenly over the cake mix, ensuring maximum coverage of dry powder. Sprinkle with granulated sugar if using.
06 - Bake for 35-40 minutes until the surface turns golden brown and edges bubble actively.
07 - Allow the cake to rest for at least 15 minutes before serving. Serve warm with whipped cream or vanilla ice cream.