- Preheat the oven to 180°C (gas 4). Put the potatoes in a saucepan, cover with lightly salted boiling water and cook for 15 minutes or until tender.
- Meanwhile, put 40g of the butter in a saucepan with the flour, 450ml milk, bay leaf and parsley stalks. Cook over a medium heat, whisking all the time until the sauce bubbles and thickens. Turn down the heat as low as possible and cook for 2–3 minutes, stirring frequently.
- Turn off the heat and remove the bay leaf and parsley stalks. Stir in the fish, prawns, chopped parsley, chives and lemon juice. Season to taste with salt and pepper, then spoon into a 1.5 litre buttered baking dish.
- When the potatoes are tender, drain them well and mash with the remaining butter. Stir in the chopped watercress and 2 tbsp milk, then beat with a wooden spoon for a minute or two until fluffy.
- Spoon the mashed potato over the fish mixture, then spread it evenly right to the edge of the dish. Fork up the surface. Bake for 25 minutes or until bubbling and browned. Serve hot.
*Leave out the prawns, if preferred, and use smoked haddock with two chopped, hard-boiled eggs instead.
*If you happen to have a carton of cream in the fridge which needs using up or plan to make this pie for a special occasion, add it either to the sauce or mashed potatoes, in place of some of the milk, for a more luxurious result.