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. Distribute mandarin orange segments over the filling and sprinkle with fresh lemon zest.
03 - Sprinkle dry yellow cake mix evenly over the fruit layer. Do not stir or mix—let it rest as a dry layer on top.
04 - Drizzle melted butter and whole milk evenly over the cake mix, covering as much dry mix as possible. For additional sweetness and crunch, sprinkle granulated sugar across the surface.
05 - Bake for 35–40 minutes until the top turns golden brown and edges bubble vigorously. The center should be set and slightly firm to the touch.
06 - Let the cake rest for at least 15 minutes before serving. Serve warm, topped with whipped cream or vanilla ice cream if desired.