Preheat your oven to 350°F (175°C). Lightly grease a 9×13-inch glass casserole dish with nonstick spray or a thin coating of soft butter so the edges don’t stick.
Spoon the lemon pie filling into the prepared glass dish and spread it into an even layer, making sure it reaches all the corners. This will be the bright, tangy base of your cake.
Sprinkle the dry white cake mix evenly over the lemon pie filling. Use your hands or the back of a spoon to gently spread it so you have a fairly even layer from edge to edge. Do not stir the cake mix into the filling.
Slowly drizzle the melted butter over the top of the dry cake mix, trying to cover as much of the surface as you can. It’s fine if a few dry patches remain; they’ll still toast up in the oven and add a little crunch.
Place the glass casserole dish on the middle rack of the preheated oven. Bake for 35–45 minutes, or until the top is lightly browned, the edges are bubbling, and the center looks set with some golden, crinkly spots.
Remove the dish from the oven and place it on a heatproof surface or a mottled granite countertop to cool slightly. Let the cake rest at least 10–15 minutes so the lemon layer thickens and the topping firms up a bit.
Serve warm, scooping out portions with a large spoon so you get some of the soft lemon filling and the crisp, buttery cake topping in each serving. Store any leftovers, if you’re lucky enough to have them, covered in the refrigerator and rewarm gently before serving.