Method

1. Heat the oil in a large saucepan. Gently fry the onion, fennel, celery and carrot for 15 minutes or until softened and golden-brown.

2. Peel the crayfish or prawns. Cut down the back of each with a small, sharp knife and remove the black intestinal track – it will look like a very thin elastic band. Put the flesh on a plate, cover with clingfilm and chill in the fridge.

3. Put all the shells and heads into the pan with the vegetables. Add the bay leaf and thyme. Cook for five minutes over a medium high heat, crushing the shells with a wooden spoon to extract as much flavour as possible. Stir often to ensure the vegetables do not burn. Stir in the tomato purée and cook for a further minute.

4. Add the wine and cook until the alcohol has evaporated. Add the water and salt and bring to the boil. Reduce the heat and simmer gently for 40 minutes, stirring occasionally.

5. Remove the pan from the heat and strain the liquid through a colander lined with muslin set over a medium saucepan. Press the shells with a ladle to squeeze out as much of the stock as possible. You should end up with around 200ml/7fl oz.

6. Bring the stock to the boil and cook until the liquid has reduced to approximately 75ml/3fl oz. Remove the pan from the heat and set aside.

7. For the trout, melt the butter with the oil in a large non-stick frying pan. Season the trout fillets on both sides with salt and freshly ground black pepper. Cook the fish, skin side down over a medium-high heat for four minutes.

8. Turn the fillets over, peel off the skin and cook for a further 2-3 minutes. Remove from the pan and keep warm.

9. Bring the stock back to the boil. Reduce the heat a little and stir in the double cream. Cook for a further minute and season to taste with salt and freshly ground black pepper. Add the shellfish and cook until the flesh is pink.

10. Divide the watercress between four warmed plates and top with the fish fillets and shellfish. Pour a little of the sauce over and put the rest into a warmed jug to serve separately.