To make the raspberry syrup, place the raspberries, sugar, water and sherry in a small saucepan and stir over medium heat until the sugar is dissolved. Bring to the boil and cook for 5 minutes. Strain the syrup and allow to cool. Add the strawberries to the syrup and toss to coat. Cover and refrigerate.
Place the flour, sugar and butter in a food processor and process until the mixture resembles fine breadcrumbs. Add the egg and vanilla and process in short bursts until the pastry just comes together. Turn out onto a lightly floured surface and work quickly with your hands to form a ball. Flatten the pastry, wrap in plastic wrap and refrigerate for 30 minutes.
Preheat the oven to 180°C (355°F). Roll out the pastry on a lightly floured surface until 3mm (f in) thick. Line a 24cm (9a in) round fluted tart tin with the pastry, trim the edges and prick the base with a fork. Refrigerate for 30 minutes. Line the pastry with baking paper, fill with baking weights and bake for 15 minutes. Remove the paper and weights and bake for a further 8–10 minutes or until golden. Set aside to cool.
Place the mascarpone, brown sugar and extra vanilla in a bowl and stir to combine. Place the cream in a large bowl and whisk until soft peaks form. Fold through the mascarpone mixture. Spoon into the tart shell and smooth the top. Arrange the strawberries on the tart and drizzle with remaining raspberry syrup. Serves 8.