For the pastry: Add the flour, salt and caster sugar to a large bowl and rub in the butter by hand until it resembles breadcrumbs. Then pour in the water and knead slightly until you have a uniform dough – it will be very very sticky and wet! Flatten the dough into a thick disc, wrap in clingfilm and place in the freezer for two hours.
For the filling: Preheat the oven to 200C/180C Fan and line a baking tray with baking paper. Then mix the raspberries, lemon zest & juice, sugar and two of the mint sprigs leaves finely chopped together in a bowl. Set aside for the juices to release.
To assemble: Remove your pastry from the freezer and roll out into a large circle directly onto a sheet of baking paper -(approx 30cm diameter) it doesn’t matter too much how even the circle is, your galette can be rustic. Then sprinkle over the chocolate chips and press them into the dough gently.
Spread the ground almonds onto the pastry in a even circle, making sure to leave roughly a 2inch gap round the sides. Then strain the raspberries (setting the juices aside) and pile them on top of the ground almonds, then fold the sides of the pastry up.
For the egg wash: Finally, beat the egg in a small bowl, and using a pastry brush wash over the pastry a few times. Sprinkle the pastry with some Demerara sugar. Bake for 35minutes until golden brown and the filling is bubbling. Remove from the oven and leave on the baking tray.
For the glaze: Mix the reserved raspberry juice with the sugar in a small saucepan and place over a high heat. Cook for a couple of minutes until you have a thickened syrup, - where you can drag a line through it with a spatula and it takes at least 10 seconds to fill in. Then using a pastry brush, glaze over the hot galette and sprinkle with the remaining sprig of mint leaves chopped.