For the pastry
Pre-heat the oven to 180˚C.
Using a rolling pin, roll out the pastry to 4mm thickness between 2 sheets of greaseproof paper, allow to rest in the fridge for 20 minutes.
Place the rested puff pastry, still between two sheets of greaseproof paper, between two flat, heavy oven trays.
Bake the pastry (still between greaseproof paper and the two oven trays) in the pre-heated oven for 25 minutes until golden brown. Remove from the oven, remove the pastry from between the oven trays and the greaseproof paper and place on a cooling rack until cool.
Cut the pastry into 12 rectangles, each approximately 3cm x 9cm, and store in an airtight container at room temperature.
For the Saint Agur blue cheese mousse
In a large bowl, using a spatula, cream the Saint Agur blue cheese into a paste.
Add the honey, lemon juice and whipping cream and whisk together. Continue to whisk until you reach firm peaks.
Place into a piping bag with a 1cm plain round nozzle, reserve in the fridge.
For the lemon butterscotch sauce
In a medium saucepan, on a medium heat, bring the sugar, glucose and water to a light blond caramel. Carefully pour in the lemon juice and lemon zest, then slowly add the whipping cream. Stir well and bring back to the boil for 1 min.
Once the mixture is thick enough to coat the back of a spoon, pour it on to a tray to cool it down.
For the caramelised walnuts
Line an oven tray with greaseproof paper.
In a medium saucepan, on a high heat, carefully bring the sugar and water to 120˚C (carefully measure with your kitchen thermometer).
Using a wooden spoon, stir the walnuts into the hot sugar and water mixture and continue to stir until they turn a golden colour. Add the salt and mix well.
Spread the nuts onto the lined oven tray and using 2 small spoons, separate each nut whilst warm to prevent them from sticking to each other.
Allow the nuts to cool completely before using. Crush just before serving.
To finish and serve
Mix together the crushed pistachios, hazelnuts and caramelised walnuts.
Lay out 8 of the puff pastry rectangles prepared earlier on a chopping board. Pipe 2 lines of the Saint Agur blue cheese mousse lengthways down each piece. Dust the remaining 4 rectangles of pastry with icing sugar, these are the tops.
To build the millefeuille, stack two layers of the pastry with the mousse facing upwards on top of each other and top with the dusted piece of pastry. Finish by placing a cube of Saint Agur blue cheese on the top of the millefeuille for presentation.
To dress the plate, drag a tablespoon of lemon butterscotch sauce across the plate. Sprinkle the mixed nuts over the sauce with a few pieces of crumbled Saint Agur blue cheese or honeycomb pieces, and place the millefeuille next to it. Serve and enjoy.