To make the pistachio filling place the milk into a medium sized saucepan and bring to the boil. Meanwhile place the egg, yolks, sugar, cornflour and pistachio paste into a large bowl and whisk together to combine.
When the milk is at temperature pour over the egg yolk mixture and immediately whisk together, to prevent the eggs from curdling. Pour the mixture back into the pan and over medium heat cook, whisking constantly, until the mixture is very thick. Scrape back into the bowl and press a sheet of Clingfilm onto the surface and refrigerate until needed.
To make the pastry place the flour, almonds, icing sugar and salt into a food processor and pulse to combine. Add the butter and pulse until the mixture resembles bread crumbs.
Add the egg yolk and the water and pulse until the mixture starts to form a ball (if the mixture is too dry add more water a tsp at a time). Tip the mixture out onto the work surface and use your hands to gently knead together into a uniform dough. Wrap in Clingfilm and refrigerate for an hour before using. (I like to form the pastry into a sausage shape before chilling and cutting into six slices to roll out individually, it’s easier to handle small pieces of pastry and it’s less likely to get warm and sticky).
Roll out the pastry on a lightly floured work surface until about 2-3mm thick and cut out 6 rounds of pastry big enough to line 10cm wide tart tins. Use the pastry to line six 10cm loose bottomed tart tins, trimming off any excess. Place the tarts onto a baking tray and refrigerate for 30 minutes or until the pastry is chilled.
Line each tart case with a piece of crumpled baking parchment and fill with baking beans or rice. Bake in an oven preheated to 180C (160C fan) for about 20 minutes then remove the parchment and the beans and bake for a further 5-10 minutes or until the pastry is golden brown.
To assemble the tarts fill each tart shell with a layer of the pistachio mixture, top with fresh raspberries and chopped pistachios, dusting with a little icing sugar if you fancy. The tarts are best served on the day they are assembled but the pastry can be made up to a week in advance and the custard up to three days in advance.
Note - You can make your own pistachio paste simply by placing 200g toasted pistachios into a food processor with a couple tbsp. of granulated sugar and processing for about 10-15 minutes until smooth and creamy.