Heat oven to 350 F and set a rack in the center. Lightly butter a 9 by 13 inch baking dish.
Spoon blueberry pie filling into the dish and spread it in an even layer. Sprinkle lemon juice and zest over the fruit.
Sprinkle the dry cake mix over the fruit. Tap the pan so the mix settles in a thin blanket.
Dot the surface with the butter pieces so they sit across the whole pan. Add nuts if you like a little crunch.
Bake until the top looks deep golden and the edges bubble about 40 to 45 minutes.
Cool 10 minutes. Serve warm with whipped cream or ice cream.