FOR THE FILLING: Gently poach the fish in enough water to cover it, together with the carrot, celery, onion and parsley for 15 minutes. Cool the fish in the stock. When it is cold, remove fish from stock and remove the flesh from the bones. Discard the skin.
Strain the stock.
Add the prawns to the stock and simmer until they turn pink (approximately 2 to 3 minutes). Remove the prawns and save the stock.
Place the fish in a shallow ovenproof dish. Arrange the prawns and the asparagus tips on top.
Make a roux by melting the butter and adding the flour to it. Cook it for 4 to 5 minutes. Heat 2 cups of the fish stock and gradually add it to the roux. Cook for 5 to 10 minutes until the sauce is smooth and thick.
Beat the sour cream and the egg yolk together, add some of the hot sauce and then in turn pour the cream and yolk mixture back into the sauce. Season to taste with salt and pepper.
Pour the sauce over the fish in the ovenproof dish.
FOR THE PASTRY: Preheat the oven to 375 F (190 C/Gas 5).
Combine the flour and the salt. Chop the butter into small pieces and mix it with the flour until it resembles fine breadcrumbs. Add the egg and lemon peel and with the minimum of kneading, prepare the dough.
Roll it out to the size of the ovenproof dish and lay it over the seafood mixture.
Brush the egg and water mixture over the pastry and bake the pie in the oven until the pastry is golden-brown and crisp.