Gratin of Lobster with Parmesan & Tarragon Sauce

• 4 cooked lobsters, weighing about
• 500 g (1 lb 2 oz)

Cut the claws off the lobster with a large knife, then split the lobster in half down the back: the easiest way to do this is to put a large knife through the centre of the body section and cut down through the head, then take out the knife, turn the lobster round and cut down the centre of the body through the tail. Remove and discard the head sac and the intestinal vein, then take out the lob