Roll (1/6th of) the puff pastry into 6 thin rectangles the size of a baking tray. Place`each piece on a baking tray and bake at 200°C for 7-9 minutes until dry to the touch and just starting to brown. Turn each piece onto greaseproof paper and allow to cool completely. The last piece, which will form the top layer of the cake bearing the chocolate covering, may usefully be made a little thicker than the others. Since you may have only one or two baking trays, do this in batches keeping a close eye on progress as the cooking time is so brief. This completes the cooking aspect of this cake.
Top and tail the lemon(s), quarter to allow pips to be removed, and then chop the entire lemon finely including the flesh, juice, zest (yellow) and skin (white) using a fine grater/ food processor thus forming a paste. Tip the paste into a large mixing bowl.
Peel, quarter and remove the core from the apples and then chop using a coarse grater/ food processor until the pieces are 2-3mm in size rather than a pureé. Add the result to the large mixing bowl and mix with the grated lemon . Add caster sugar to taste (approx 400g) followed by the double cream, mixing thoroughly, the result being a 'wet' lime green coloured textured paste which will flow off a tilted spoon. Since the cake topping will be sweet in flavour, the apple/lemon mixture should not be over-sweetened but, rather, left a little tart/sharp as a foil to the taste of the topping.
Place a piece of the cooked pastry onto a cake board(1) and spoon some of the paste onto it, smoothing it into a layer 2-3 mm thick. Place another piece of the cooked pastry on top and press out any bubbles (puff) in it and add another layer of paste. Continue thus until the last piece of cooked pastry is placed on top but do not cover this last piece with the paste as this will bear the chocolate covering. Trim the edges square, using the surplus to fill any voids in the side of the cake.
Break up the chocolate flavoured covering into a saucepan and, if desired, add water to half fill the voids in the chocolate pieces. Warm the chocolate on the hob, stirring continuously, until bubbles form in the mixture. Spoon onto the top of the cake and drizzle down the sides, wiping any spills back onto the side of the cake with a knife/spatula.
Allow to stand in a cool place overnight, or 24 hours, for the topping to solidify and the body of the cake to blend together. Serve on side plates with a fork.
(1) When selecting a cake board bear in mind that from this point onwards you will not be able to lift the cake for relocation.
(2) I once made this cake after purchasing some time-expired eating (roseate) apples from the supermarket at 9p per bag - less than 1p per apple! Using 2Kg of eating apples the amount of sugar required to adjust the taste to 'slightly tart' was very small indeed - about 5 tablespoons full. The cake worked out surprisingly inexpensive.
(3) If catering for a group which includes 1 or more celiacs obviously use gluten-free puff pastry or make your own using gluten-free flour and baking powder. Read any warnings on the chocolate covering lest this be made in an environment which is not gluten-free.