Lightly grease a baking sheet. Roll out the pastry to a 30 cm (12 inch) disc. Place on the baking sheet and roll the edges up to create a 1 cm (1/2 inch) border.
Press the border down with your thumb to make a crumpled edge, then prick over the middle of the pastry disc a few times with a fork. Place in the freezer for a few minutes.
Brush the border of the pastry with a little of the beaten egg. Mix together the mascarpone, remaining egg, Parmesan and basil, season to taste and spread over the centre of the tart. Top with the tomatoes and drizzle over the oil.
Place in a preheated oven, 220°C (425°F), Gas Mark 7, for 20–25 minutes until golden and crisp.