Sea trout are a rare treat. They are genetically identical to brown trout, but live part of their lives in salt water, growing big and sweet-fleshed in the process. You could use small wild or organic salmon or large trout instead – or even fillets and steaks.

Serves 4-6

1 sea trout, about 1½ kg, cleaned and gutted
2 tbsp sunflower oil
Salt and pepper
1 lemon, juiced, with the peel sliced into rings
1 bay leaf

For the green mayo

2 handfuls of watercress
Small handful of tarragon
Small handful of chives
2 egg yolks
1 tsp dijon mustard
200ml olive oil (not extra-virgin), at room temperature
Salt and pepper
1 tbsp lemon juice, or maybe a little more
1 heaped tbsp crème fraîche

Preheat the oven to 190C/375F/Gas Mark 5. Rinse the fish inside and out and pat dry. You don’t want it to be fridge-cold, so let it warm up a little. Cut a piece of foil twice as long as the fish and wide enough to wrap around it 1½ times. Lay the foil in a roasting tray so that it overhangs both ends.

Rub the oil over the fish and season the cavity. Place in the middle of the foil. Squeeze over the lemon juice, then tuck the lemon rings into the belly with the bay leaf. Pour in 125ml water and season again. Gather the foil to form a loose parcel, then fold to seal it securely, but leave room inside for the steam to circulate: you want the fish to cook in its own juices.

Place in the preheated oven and cook for 10 minutes per 500g. (This produces fish that is fashionably just cooked in the middle; add a few minutes extra cooking time if you want it better done.) Remove from the oven and allow to cool in the foil, checking it is done at the thickest point after 10 minutes. If you’re happy, undo the foil completely. When cold, slice through the skin along the centre line and peel away to uncover the coral-pink flesh.

Meanwhile, make the green mayo. Put the watercress in a food processor with the herbs and whizz for a couple of seconds until finely chopped.

Put the egg yolks and mustard in a large bowl, then whisk together. Start to whisk the oil into the yolk mixture, drop by drop at first, until it starts to thicken. Gradually whisk in the remaining oil, in a steady stream, until the mixture is the thickness of stiffly whipped cream. Season and add the lemon juice. Add to the food processor with the herbs and crème fraîche, then quickly whizz to combine. Taste and season again, adding more lemon juice if needed.

Lift the fillets off the fish, then remove the backbone and lift the bottom fillets from the skin. Eat with the mayo, some buttered new potatoes and the simplest of cucumber salads.

Times Recommends

• How to season a wok
• Chicago Rib Shack hits back at its critics
• Gordon Ramsay: Indian barbecue