Preheat oven to Gas 6, 200ºC, 400ºF.
Line a large baking tray with non-stick baking paper.
Onto the tray place the sugar and spread out into a rough circle about the size of a large dinner plate. Break the butter into small pieces and add. Now add the onion, apple, rosemary and lamb cutlets – remembering that the bottom of the tart will become the top when turned out after cooking!
Place in a hot oven for about 10 minutes, to start with.
Roll out the pastry to a similar size to the tray. Remove tray from oven and carefully place the pastry over the cutlets, return to the oven for about 20 minutes until the pastry is a good dark golden colour.
Remove from the oven and after a couple of minutes carefully turn out onto a large plate.
Serve in large wedges.