For the egg glaze: 1) In a small bowl combine the egg and water. Put aside.
For the crust: 1) Preheat oven to 210 degrees C/Gas 6. Place rack on lowest level.
2) Pulse plain flour, butter, sugar, and salt in food processor until butter looks like peas.
3) Whip egg and add egg yolk, water and vinegar. Slowly pour mixture in while pulsing until mixture will hold together when squeezed. Cut off 1/4 for stars, roll into ball and refrigerate.
4) Roll rest of dough immediately out to 33-cm circle, no more than 0.5-cm thick. Put in a 23-cm pie dish and press sides and bottom toward bottom ridge. Trim overhang to 2-cm, flute edges.
5) Freeze crust in shell for 5 minutes. Weigh down crust, lined with baking paper with pie weights (beans etc).
6) Glaze edges with egg white glaze and bake crust on a baking tray. Bake on bottom shelf of oven for 15 minutes. Cover sides of crust with foil as soon as they set and start to turn golden. Rotate crust in oven half way through. Remove pie weights from pie shell after 15 minutes. Prick bottom with fork. glaze bottom and sides with egg white. Bake for 10 more minutes on bottom rack without baking sheet.
For the star shapes: 1) Roll out remaining crust, cut into decorative star shapes. Glaze with egg mixture, sprinkle with sugar and place on sheet of foil. Bake at 210 degrees C/Gas 6 until slightly golden, then remove.
For the filling: 1) Place blueberries in a medium pot over high heat. Add cherries and sugar. Stir until mixture is thawed and juicy. Add lemon juice, lemon verbena, almond essence , butter and salt and 2 tbsp plain flour.
2) Bring mixture to a boil. Reduce heat to low and cook for at least 15 minutes to cook plain flour and thicken juice. Add additional plain flour, if needed, to thicken further before putting in pie shell. Remove from heat.
For the streusel: 1) Put plain flour, sugar and salt in processor and pulse 2 times. Add walnuts. Mix melted butter and vanilla together. Add to processor and pulse. Add cold butter and pulse.
For the assembly and baking: 1) Add filling to shortcrust and top with streusel.
2) Bake at 210 degrees C/Gas 6 on lower third of rack for 25 to 30 minutes or until streusel is browned and set. Rotate pie half way through baking.
3) Remove pie and cool in freezer or on dry ice for 30 to 35 minutes.
4) Decorate top with stars and let sit 2 minutes before cutting.