TO MAKE STOCK
Place the fish bones in a pan, pour in enough water to cover, and add bouquet garni, sliced carrot and onion. Bring to the boil and simmer gently for 30 minutes; strain and set aside.
Before you start: You will need a 17,5 cm diameter savarin, or ring mould. Grease the ring mould well and set the oven at 170 ºC.
1. Trim the fillets of sole, wash and dry thoroughly and leave wrapped in absorbent paper until needed. Skin and bone the hake or haddock and place in a food processor. Check the weight of the minced fish: you need at least 340 g but no more than 450 g.
PANADE: Sift the flour onto a piece of paper and set aside. Cut the butter into pieces, put in a saucepan, add milk and bring slowly to the boil. Remove the pan from the heat and tip in the sifted flour all at once. Heat until the mixture is smooth. Work the panade, beaten eggs and minced fish together and when thoroughly mixed and smooth, stir in the sour cream, seasoning and dill.
2. Line the ring mould with the fillets of sole, skinned side uppermost, overlapping slightly so that there are no gaps. Fill the lined mould with the fish panade and fold the ends of the fillets over the top. Cover with a piece of thickly buttered greaseproof paper and bake in a bain marie for 40 to 45 minutes, or until the fillets of sole are firm to the touch.
3. GARNISH and PRAWN BUTTER: Peel away the shells of the prawns, and put the flesh between two buttered plates and reheat. Trim and wipe mushrooms, place in a small pan with 15 g butter, salt and black pepper. Sauté mushrooms lightly and set aside. Pound prawns and 30 g butter together until smooth. Push prawn mixture through a metal sieve and set aside.
4. VELOUTÉ SAUCE: Prepare as for a white sauce. Stir until sauce begins to thicken, add the cream and bring to the boil. Cook for 2 to 3 minutes, draw aside and stir in prawn butter, season well. Reheat prawns and mushrooms for a minute or so.
TO SERVE: Turn out mould onto a hot serving dish and leave for 2 to 3 minutes. Wipe any cooking juices with a clean cloth. Coat with velouté sauce, fill centre with watercress, mushrooms, prawns and lemon slices.