Pulse flour, sugar, salt, vanilla seeds, and butter in a food processor until combined.
Add beaten egg and milk, pulsing until dough forms a ball.
Wrap dough in cling film and refrigerate for at least one hour.
Preheat oven to 180°C.
Roll out pastry thinly and line a 23cm flan dish.
Blind bake with parchment and beans for 30 minutes until golden.
Boil milk with a vanilla pod and set aside to infuse.
Whisk egg yolks and sugar until fluffy, then stir in flour.
Whisk hot milk into the egg mixture and cook over medium heat until thickened.
Strain crème pâtissière through a sieve and chill with cling film touching the surface.
Fill the pastry case with cooled crème pâtissière and smooth the top.
Arrange strawberries, raspberries, blackberries, and redcurrants over the filling and chill.
Heat half the champagne with sugar, dissolve softened gelatine, and stir in remaining champagne.
Cool jelly until nearly set, then spoon over the fruit and chill until firm.