Start by making the custard filling. Heat the milk with the pistachio paste. Whisk the egg yolks in a separate bowl with the sugar, flour and cornflour until smooth. When the milk is steaming, just before boiling, gradually pour it over the yolk mixture and whisk thoroughly. Return the mixture to the heat and cook until thickened and starting to boil. It will go very lumpy, but keep whisking vigorously until smooth. Pour into a clean bowl. Cover and leave to cool completely. The pistachio custard can be made up to two days in advance and kept in an airtight container in the fridge.

Meanwhile, make the choux. Heat the milk, 125ml water and the butter in a large saucepan and bring to the boil. Take off the heat and add the flour, sugar and a pinch of salt. Mix and return to the heat, stirring with a wooden spoon until the mixture is smooth and comes away from the sides of the pan. Tip into a tabletop mixer and mix with the paddle attachment until the mixture has cooled (if you don’t have a mixer, spread out on a plate to cool). Once cooled, start gradually beating in the egg (either in the tabletop mixer or with an electric hand whisk) until fully incorporated. You want the mixture to be glossy and just fall off the spoon in 3-5 seconds. Put in a piping bag and set aside.

Heat the oven to 220C/200C fan/gas 7. Line two baking trays with baking parchment. Snip about 1cm off the end of the piping bag and pipe the choux pastry down in a 10cm line, then back on itself, leaving space between the eclairs for the choux to expand. You may need to do this in batches. Bake for 3-4 mins, then reduce the heat to 180C/160C fan/gas 4. Bake for 12-15 mins more until golden all over and cooked through. Remove, pierce the side of each one to let out the steam, then cool them on a wire rack. Can be made a day ahead and kept in an airtight container.

When ready to assemble, whip the cream until soft peaks form, then set aside. Using electric beaters, whisk the custard mixture until smooth. Fold in the whipped cream using a spatula and put the custard in a piping bag.

To serve, cut the choux pastry in half horizontally and fill with blobs of the pistachio custard. Using a fine sieve, liberally dust the tops with icing sugar and sprinkle with stem ginger and chopped pistachios just before serving.