Scatter some grated Parmesan (if used), onto a large baking sheet, then put the rolled pasty on to top and unroll, making sure the bottom side is coated all over with Parmesan.
Using a spatula or silicon brush, spread evenly a thin layer of pesto on top of the pastry.
With a blunt knife draw a line across the centre of the pastry sheet (horizontally). Scatter some more Parmesan on top, if you like.
Carefully roll up one horizontal side to the centre line, then the other side. Cover with clingfilm, then chill for at least 15 minutes, until needed.
Preheat the oven to 190°C.
Cut the roll into 1 cm slices and place them onto a lined baking sheet. (You can brush them with egg-wash or milk, if you’re not vegan.)
Bake for about 8 minutes, until golden, then flip them and bake for another 5 minutes or so.
Serve freshly baked or at room temperature.