Into a bowl, add the packets of lemon jelly powder, add the very hot water, and fully dissolve it. Then add the cold water and mix it all together very well.
Transfer to a large glass dish and leave in the fridge until firm.
Into a bowl, add the condensed milk, the two packs of table cream and the lemon juice, mix it all together very well until combined, add the unflavored gelatin previously dissolved and mix for about 5 minutes.
Pour this cream over the jelly into the glass dish.
Take it to the fridge again and leave it for about 3 hours.
Top with some lemon zest and garnish to taste.
Help yourself to a slice.