Bring a large pot of water to the boil and blanch the asparagus for about 1 minute. Remove from the boiling water and lay on a plate to cool. Open the pastry sheets and remove one at a time from the packet. Cut in half and brush lightly with butter. Sprinkle on the grated cheese, a few chilli flakes if using and season with salt and pepper. Lay a spear of asparagus along the edge of the pastry and roll it as tightly as possible. Place on a baking tray, you may need a couple of trays. Brush again with a bit more melted butter. Repeat with the remaining asparagus. Place the trays into a 400 degree oven and bake for about 7-10 minutes until golden brown.