For the candied lemon, place the sugar in a pan with 200ml cold water and heat until the sugar dissolves. Add the lemon slices and essence, cover the surface with baking paper, reduce the heat to low and simmer for 30 minutes, or until the lemon is almost translucent.
Remove from the heat, leave to cool, then remove the lemon slices to a wire rack over a tray and set aside.
Place 200ml boiling water in a large saucepan over a low heat, sprinkle in the gelatine and whisk for 2–3 minutes, until it dissolves. Add the lemon curd and stir for a few minutes, then remove from the heat and leave to cool.
Line a 24cm spring-form cake tin with clingfilm so it overhangs the sides. Place 1 crêpe in the base of the tin and spread over 2 tablespoons of lemon curd mixture.
Continue layering the crêpes and curd until you've used all the crêpes, finishing with a crêpe. Pull the clingfilm over the cake and chill overnight, or until the cake is firm.
Remove the cake from the tin and transfer to a cake stand or large plate. Top with the candied lemon slices and a drizzle of the syrup, then scatter over the lemon zest and serve with a dollop of crème fraîche.