Heat oven to 180C/160C fan/gas 4. To make the pastry, put the flour, sugar and a pinch of salt in the bowl of a food processor, then pulse to combine. Add the butter and pulse until the mixture resembles coarse breadcrumbs. Add the egg yolk and 3 tbsp very cold water, and pulse until the mixture just starts to come together. Tip the dough onto a work surface and use your hands to bring together into a uniform dough. Wrap in cling film and chill for 1 hr. Can be made up to 3 days in advance.
On a lightly floured work surface, roll out the pastry and use to line a 23cm tart tin. Trim off the excess and chill for 30 mins. Prick the chilled pastry shell with a fork and line with a crumpled piece of baking parchment. Fill with baking beans or rice, and bake in the oven for 20 mins. Remove the parchment and put the pastry back in the oven for 12-14 mins or until golden. Take out of the tin and put on a wire rack to cool completely.
To make the filling, put the mascarpone, cream, vanilla and icing sugar in a large bowl and whisk until it just holds soft peaks. Spread the cream mixture into the pastry case in an even layer. Decorate with the fruit, however you like. Best eaten on the day.