Melt 25 g (1 oz) of the butter in a saucepan, add the onion, leek, celery, garlic, lemon rind, tarragon and salt and pepper to taste and cook gently for 10 minutes until soft.
Add the cream and bring to the boil, then reduce the heat and simmer gently for 2 minutes until thickened. Remove from the heat and stir in the fish and prawns.
Spoon the seafood mixture into a 1 litre (1¾ pint) pie dish. Melt the remaining butter in a small saucepan. Arrange the bread slices over the fish, overlapping, and brush with the melted butter.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15–20 minutes until the bread is golden. Cover with foil and bake for a further 10 minutes until the fish is cooked.