Trim the crusts from the bread to make neat squares. Remove some of the crumb from each slice, leaving a hollow in the centre. Melt 1 oz / 30 g of the butter, mix it with the oil and brush over the bread squares. Place on a baking sheet. Peel the prawns, reserving the shells, and put the flesh to one side. Sweat the prawn shells with the shallot, carrot and celery in half the remaining butter. When the vegetables are soft, add the tomatoes and herbs. Pour on the wine and water, and let it all bubble gently for 20 minutes or so. Strain it into another saucepan, and boil to reduce by half. Meanwhile, bake the prepared bread squares in the oven preheated to 200°C / 400°F / Mark 6 for 8–10 minutes or until crisp and golden, and keep them warm on a plate. Add the cognac to the sauce, and flame it before stirring in the cream. Whisk in pieces of the remaining butter, and season to taste. Stir in the prawns, let them heat through, and then spoon the sauce into the toasted bread squares. Sprinkle with the grated Gruyère, and brown under a hot grill. Serve immediately.