Mix the cooled vegetables with the ricotta, egg yolk, Grana Padano and lemon zest until well blended. Season with a few gratings of nutmeg, salt and freshly ground black pepper. Preheat the oven to 200˚C. Lightly beat the egg with a pinch of salt. Cut the puff pastry into 8 (12x23cm) rectangles (you will have some left-over pastry). Line two large baking trays with baking paper and lay two pastry rectangles on each one. Spread a quarter of the vegetable and ricotta mixture on top of each rectangle, leaving a 2cm border all around.