Step 1

To make the mayonnaise, put the egg yolks in a bowl with the mustard and garlic, and mix well. Start beating with an electric beater or wooden spoon, then gradually add a few drops of a mixture of both oils. Increase the stream of oil as you whisk, making sure the mixture is thickening. Only add more oil once the previous lot has been incorporated. If it separates, see the my tip (right). Add the vinegar, lemon juice and seasoning to taste.

Step 2

For the sauce, bring a large saucepan of water to the boil and blanch the spinach and watercress, then plunge them into cold water to keep their colour. Drain. Squeeze excess water out of the spinach and watercress. Put all the greenery, including the herbs, in a food processor and blitz. Add the mayonnaise and blitz again. Add the lemon juice and seasoning to taste.

Step 3

Preheat the oven to 240°C, gas mark 9. Wash the salmon inside and out, and pat it dry with kitchen paper. Make sure to wash any bloody bits well as they make the salmon bitter. Melt the butter.

Step 4

Put a large sheet of foil – big enough to enclose the salmon with room to spare – on a rimmed baking sheet or in a large roasting tin. Brush the foil – in the middle only – with some butter. Season the fish inside and out, and put it on the buttered foil. You will probably have to lay it diagonally. It doesn’t matter if the tail and the head don’t fit completely inside the tin.

Step 5

Put the herbs inside the fish, then drizzle the rest of the butter over and inside. Pour on the wine and add the lemon juice. Tent the salmon with the foil by scrunching the edges together. Bake for 45 minutes. It should be ready, but to check, rub the skin with the tip of a blunt knife: if it slips off easily, the salmon is done.

Step 6

Remove the skin, being careful to keep the flesh intact, but leave the head and tail on. Serve the salmon whole on a platter (be careful not to lose the cooking juices when you move the salmon – you can spoon a little onto each serving). You can also serve the salmon in pieces on individual plates. Offer the sauce verte on the side.