01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish 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 filling. Sprinkle fresh lemon zest across the fruit layer.
04 - Sprinkle dry yellow cake mix evenly over the fruit layer. Do not stir or mix the layers.
05 - Drizzle melted butter and whole milk evenly over the cake mix, covering as much dry powder as possible. Sprinkle with granulated sugar if using.
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.