Heat oil in a saucepan and fry the onion, fennel, celery, and carrot for 15 minutes.
Peel the crayfish and remove the intestinal track before chilling the flesh in the fridge.
Add shells, heads, bay leaf, and thyme to the pan and cook for 5 minutes while crushing shells.
Stir in tomato purée, then add wine, 300ml water, and salt, simmering for 40 minutes.
Strain the stock through a muslin-lined colander into a saucepan to obtain about 200ml liquid.
Boil the stock until it reduces to approximately 75ml and set aside.
Melt butter with oil in a frying pan and cook seasoned trout fillets skin side down for 4 minutes.
Turn the fillets, remove the skin, and cook for 2 to 3 minutes until done.
Boil the reduced stock, stir in cream, and cook the crayfish flesh until pink.
Serve the trout on a bed of watercress and top with crayfish and sauce.