Prep time 240 minutes
Cook time 60 minutes
Yield: Serves or Makes 12
For the pie crust, place vinegar in a jug with 1/2 cup (125ml) cold water and 4 ice cubes. Place flour and sugar in a bowl with ½ teaspoon fine sea salt. Add the butter and toss to completely coat in flour. Turn out onto a clean work surface and using a pastry cutter, cut the butter into the flour until coarsely combined (leave some large chunks of butter in the mixture, as this will ensure a flaky pie crust). Make a well in the centre and add sour cream, then add 2 tablespoons of vinegar water at a time, carefully folding the flour and butter into the water, until all the water has been used up (discard the ice cubes). Divide into two pieces and enclose in plastic wrap. Chill for at least 3 hours or overnight.
Roll out one ball of pastry to1/8-inch thick and use to line the base and sides of a 7x11-inch rectangular pie dish. Chill for 30 minutes. Roll out second ball of pastry to 1/8-inch thick and cut 2 x3-inch thick strips. Chill for 30 minutes.
Combine rhubarb, sugar, orange zest and juice in a pan over medium heat. Cook, stirring occasionally, for 15 minutes until rhubarb is soft and most of the liquid has reduced. Remove from heat and cool completely.
For the frangipane, beat butter and sugar in an electric mixer until pale. Beat in sour cream, egg, yolk and vanilla until combined, then beat in pistachio and flour. Spread into prepared pie dish then top with rhubarb mixture. Arrange pastry strips in a lattice pattern over the top, then fold over the edges to seal, trimming excess from the strips. Brush with egg wash and scatter with demerara sugar. Chill while oven is preheating.
Preheat oven to 400˚F. Place pie on a baking tray to catch any jam overflow and bake for 45 minutes or until golden and bubbling. Rest for 30 minutes before serving.