First prepare the lobster meat. Remove the claws and legs away from each of the lobsters. Using a large knife cut the lobsters in half. Clean out the head cavities and place the contents into a small bowl with the legs.

Carefully remove the dark tub from the tail meat and discard. Chop the lobster tail and put into another bowl with the cooked prawns. Rince out the shells and set aside.

Next carefully crack the claws and knuckles with the knife, then pick out all the meat using a lobster pick or small skewer. Cut up the claw meat and add all the lobster to the bowl with the tail meat.

Place any shell into the bowl with the legs and head meat.

To make the sauce, melt the butter in a medium sized heavy-bottomed saucepan over a medium heat and soften the shallots for fifteen minutes until cooked and golden.

Add the retained head meat and lobster shell and turning up the heat and cook for couple more minutes stirring constantly. Add the brandy and vermouth and carefully flame.

Add the mustard, béchamel, cayenne, cream, and tarragon stems and bring to a gentle bring to a gentle simmer, cook the sauce for twenty minutes stirring regularly to prevent sticking. Remove from the heat and leave to cool slightly.

Pour the cooled sauce into another saucepan through a fine mesh sieve and push with the back of a sturdy wooden spoon to get all of the sauce. Season with lemon juice, salt and pepper. This can be made in advance, covered, and kept chilled.

To serve, carefully reheat your sauce over a low heat until warmed through, then stir in the reserved lobster, prawns and chopped tarragon. Place the shells on to a baking tray and spoon in the mixture, then sprinkle with the cheese and place under a preheated grill for until the sauce is bubbling and the cheese golden brown. Carefully remove and place on a plate with a fish slice.