Grana Padano, Potato and Asparagus Tart

1. Preheat oven to 200ºC, (400ºF / Gas Mark 6)
2. Cook the potatoes in a pan of lightly salted water for 5 minutes until just tender, adding the asparagus for the last 2 minutes. Drain and allow to cool slightly.
3. Unroll the pastry sheet and place on a large non-stick baking sheet. Using a sharp knife score a 2.5cm (1") rim along the sides of the rectangle being careful not to cut all the way through. Prick the inside area lightly with a fork.
4. In a bowl, beat together the crème fraîche, lemon zest and seasoning until well combined.
5. Spoon this mixture inside the marked edge and sprinkle over three quarters of the Grana Padano cheese. Scatter over the potatoes and asparagus and sprinkle over the remaining Grana Padano cheese. Cook in the centre of the oven for 25 minutes until the pastry has risen and is golden and the filling cooked.
6. Serve topped with some rocket leaves and shavings of the Grana Padano cheese. Serve sliced.