First, prepare your apples. Place the slices in a saucepan with the water, sugar and lemon juice and toss to coat. Cover and simmer over medium heat 10 minutes or until soft but not squishy, stirring occasionally, about 10 minutes. Drain well and let cool.
Thaw your puff pastry sheets by putting them on the counter for about 30 minutes. Meanwhile, preheat oven to 200 degrees C.
Line a 20cm square baking tin with baking paper. Also line a larger baking tray with baking paper.
When pastry has thawed, trim the sheets to the size of the 20cm baking tin (remember the pastry will shrink just a little when baking). Place one pastry square on the larger baking tray and place the square tin on top (this helps it not rise too much). Place the other pastry sheet in the square tin.
Bake until puffed and golden, about 15 minutes. Let cool and remove the baking paper.
Make the pudding according to packet direction, but don't use all the required milk. If it asks for 3 cups milk, use 2. This will help it set firmer.
Place the flatter square of pastry in the square tin. Top with apple slices and distribute evenly. Sprinkle raisins evenly over top.
Pour custard over the apples and top with the other puff pastry sheet, pressing down lightly. Refrigerate 3 hours before cutting into 6 or 9 slices. Ice if desired.