Turn the grill to high, put the pittas on the grill rack and heat for about a minute while you slice the mozzarella into five.
Turn the pittas over and spread each one with 1 tsp pesto, then top with a mozzarella slice. Pile the ham on top, so it looks quite ruffled, then tear the final mozzarella slice into four, put it on top of the ham and grind over black pepper. Return to the grill for 3-4 minutes more until melted and starting to turn golden.
