Begin by making the sweet shortcrust pastry. This can be done in a food processor or done by hand. Place the plain flour into the bowl of the processor and add a pinch of Maldon salt and the cold, cubed butter. Pulse this until resembles fine breadcrumbs (try not to overmix). Mix in the icing sugar. Next add the egg yolk and pulse again until it forms a dough.
If you are doing this by hand, instead use your fingertips to rub the butter into the flour.
Tip the dough onto some clingfilm and press it to a disc shape and then wrap in the clingfilm and place into the fridge to chill for 20 minutes.
Preheat the oven to 180C.
After 20 minutes, remove from the fridge. Lightly dust a work surface with plain flour and then use a rolling pin to roll out the pastry to a large circle, roughly 1cm thick and large enough to fill a 20cm tart tin. Lift and line the tart case with the pastry, making sure to press it well into the corners. Use the rolling pin to roll off the excess across the top. Lightly prick the base of the pastry with a fork. Then use some baking parchment and baking beans to line the base of the filled tin ready to be blind baked.
Place the tart tin into the middle shelf of the preheated oven and bake for 15 minutes, then remove the paper and baking beans and return to bake for a further 10 minutes. It should be pale golden and chalky to the touch.
Remove the tart from the oven and set to one side. Now make the pistachio frangipane filling. Blitz the pistachios till the are fine. In a food processor or using electric hand whisks, beat the softened butter with the sugar and add a pinch of Maldon salt. Beat until the mixture is pale and light and fluffy. Next add the eggs and beat again. Then add the blitzed pistachios, ground almonds and 1 tbsp of plain flour and fold through until evenly mixed.
Tip this frangipane into the baked tart case and smooth over the top. Dot the cherries across the top and very lightly press them in. Sprinkle over the flaked almonds and then place into the oven to bake for 40 minutes until lightly golden on top and the frangipane has puffed up.
Remove from the oven and allow to cool for at least 20 minutes before slicing and serving dusted with icing sugar.