FOR THE WATERCRESS SAUCE:

1. Blanch the watercress in boiling water for 30 seconds to remove some of the bitterness, then refresh in iced water.
2. Place in a large colander and squeeze out as much water as possible.
3. Chop and transfer to a blender with the olive oil, water and a pinch of salt and pepper.
4. Blend on full speed, then taste and correct the seasoning if necessary.
5. Pass through a fine sieve and warm in a pan when ready to serve.

FOR THE POTATOES:

1. In a large saucepan on a medium heat, bring the potatoes, water and salt to a boil.
2. Turn down the heat and cook for 12 minutes on a gentle simmer then turn off the heat and leave in the cooking liquor until the trout is ready.

FOR THE TROUT FILLETS:

1. first pat the fish dry with kitchen paper. Season lightly with salt and pepper.
2. In a large non-stick frying pan on a medium heat, cook the butter until it starts to turn a nutty hazelnut brown.
3. Place the trout in the pan, skin-side down.
4. Very fresh fish will arc upwards at this point so, using a fish slice press the fillets down for a few seconds so they keep contact with the pan and stay flat.
5. Cook the trout fillets for 7–8 minutes depending on their thickness, until the skin is crisp.
6. You should hear just a gentle sizzle as the skin browns and crisps beautifully and there should be no smoke rising from the pan.
7. Too much heat will overcook the fish and the butter will burn.
8. Turn the fillets over on to the flesh side and cook for a further minute.
9. Remove the trout from the pan and leave to rest for 1 minute on a warm plate.
10. Stir the water and lemon juice into the pan juices.

TO SERVE:

1. Place a small pile of the cooked potatoes in the centre of each plate, brush the sea trout with the pan juices and place on top of the potatoes.
2. Drizzle the warmed sauce around the plate and finish by scattering over a few watercress leaves.