Thaw puff pastry, brush with a little water and roll out into two rectangles of about 12 x 20 cm (approximately 5 x 8 inches). Place puff pastry on two buttered ovenproof dishes of appropriate size. Rinse blueberries, trim and dry on absorbent paper towel. Evenly distribute blueberries on puff pastry. Reserve a handful of blueberries for garnish.
Preheat the oven to 170°C (approximately 350°F). Cut vanilla bean in half lengthwise and scrape out seeds. Mix mascarpone with sugar, lemon zest, vanilla seeds and double cream, spread evenly over blueberries and bake for 12-15 minutes in preheated oven. Serve immediately garnished with fresh blueberries.