Beat the butter and sugar together until smooth, but not fluffy. Mix in the egg until combined, then add the flour. Bring together with your hands to form a dough. Wrap and chill in the fridge for 30 mins.
Dust a surface with flour and roll out the pastry to a 26cm circle (or 3cm bigger than your tin). Line a 23cm fluted tart tin with the pastry, leaving an overhang. Prick the base with a fork to stop any air bubbles forming. Chill for 30 mins.
Heat the oven to 200C/180C fan/gas 6. Line the pastry case with a scrunched up sheet of baking parchment and baking beans, then bake for 15 mins. Remove the parchment and beans and bake for 15-20 mins more until crisp and golden. Leave in the tin to cool.
To make the crème pâtissière, heat the milk, cream and vanilla in a pan over a medium heat, stirring occasionally. Bring up to a simmer. Meanwhile, whisk the yolks and sugar in a bowl for 3 mins until pale, then stir in the cornflour until combined. Pour a quarter of the hot cream over the egg mix, whisking continuously, then pour the warm egg mix into the pan with the rest of the cream. Stir over a low-medium heat for 5-8 mins until the crème pâtissière thickens. Transfer to a large bowl and cool for 15 mins, whisking occasionally. Gradually whisk in the butter until smooth, cover and put in the fridge until needed.
Beat the strawberry jam to loosen it, then spread over the bottom of the pastry case. Put the crème pâtissière into a piping bag and pipe in a spiral on top of the jam. Arrange the strawberries on top in a pattern. Heat the apricot jam in a small pan with 1 tbsp water until warm. Whisk to loosen the jam, then leave to cool slightly. Brush over the top of the strawberries.