Preheat the oven to 180°C (160°C fan oven). Roll out the pastry on a lightly floured surface until big enough to line a shallow, 20cm loose-bottomed tart tin. Trim the edges to neaten. Line the pastry with foil or greaseproof paper, fill with baking beans and bake for 25 minutes. Remove the paper and beans and cook for 10 minutes more until golden.
Mix the raspberry jam and berries with the cornflour and spread over the tart. Put back in the oven for 20 minutes until fully cooked
Lift from the tin, dust with icing sugar and enjoy cold.