To store, cover the cooled casseroles tightly with plastic wrap or lids and refrigerate for up to 3–4 days. Reheat individual portions in the microwave for 20–30 seconds or warm the whole dish in a 300°F oven until heated through.
To tweak this recipe for your schedule or pantry, you’ve got options. If you can’t find lemon cake mix, use a yellow or white cake mix and stir 1–2 teaspoons of lemon zest into the pie filling before layering; a splash of lemon juice (about 1 tablespoon) also brightens the flavor. For a slightly less sweet version, use one can of blueberry pie filling and bulk it up with 2–3 cups of fresh or frozen blueberries instead of two full cans. If you prefer a richer topping, cut cold butter into thin slices and lay them evenly across the dry cake mix instead of using melted butter—this gives you more of a cobbler-style top. You can also add 1/2 cup of chopped nuts (like sliced almonds or pecans) over the cake mix before baking for extra crunch. To make this more brunch-friendly, bake one dish a bit less so it stays softer and more spoonable, almost like a fruit bake. And if you’re planning ahead, assemble both casseroles earlier in the day, cover, and refrigerate; just pull them out while the oven preheats and add 5–10 extra minutes to the bake time to account for the cold dishes.
