The way I cook a salmon is extremely slowly, wrapped in kitchen foil in the oven. I cannot recommend this method highly enough. Unfortunately, the gas equivalent of 250°F (130°C), which used to be gas mark ½, no longer exists. So, if you use a modern gas cooker that begins at gas mark 1, give the fish 25 minutes less cooking time.

Serves 8
4 lb (1.8 kg) fresh whole salmon, cleaned and gutted
2 oz (50 g) butter, plus a little extra for greasing
1 small onion, peeled and thinly sliced
3 bay leaves
4 sprigs chopped fresh tarragon
salt and freshly milled black pepper

For the sauce:
2 large eggs
2 teaspoons mustard powder
1 fat clove garlic
½ pint (275 ml) groundnut or other flavourless oil
2 dessertspoons white wine vinegar
3 oz (75 g) spinach
1½ oz (40 g) watercress, tough stalks removed
1½ oz (40 g) fresh flat-leaf parsley
2 tablespoons chopped fresh tarragon
1 tablespoon freshly snipped chives
1 tablespoon (or more) lemon juice, to taste
salt and freshly milled black pepper

You will also need a baking tray measuring 11 x 6 inches (28 x 40 cm), and some kitchen foil.

First make the sauce. Break the eggs straight into your blender or food processor, then sprinkle in the mustard powder and add the garlic clove and a teaspoon of salt. Next, measure the oil into a jug, switch the machine on and pour in the oil in a thin, steady trickle with the motor running. (You must be very careful here: too much oil too quickly and the sauce will curdle.) When all the oil is in, blend in the vinegar.

Now rinse the spinach, watercress and parsley under a little cold, running water and put them into a saucepan. Then, with the heat turned to medium, stir them around until everything has just wilted. Next, tip them into a colander and rinse them again under cold, running water to keep their colour. Squeeze out the excess moisture very carefully, thoroughly pressing with a wooden spoon. Then transfer the leaves to the blender or food processor, along with the tarragon and chives, and whiz until the sauce is smooth and green. (There will be some fine specks, but that’s okay.) Now taste and season with the lemon juice and freshly milled black pepper, and some more salt, if it needs it.

When you are ready to cook the salmon, pre-heat the oven to 250°F (130°C). (For the gas mark, see introduction). Start by wiping the fish with some damp kitchen paper, then place it in the centre of a large, double sheet of kitchen foil that you’ve generously buttered. Put 1 oz (25 g) butter, the onion slices, bay leaves and tarragon into the centre cavity, along with a seasoning of salt and freshly milled black pepper. The rest of the butter should be smeared over the top of the fish. Now wrap the kitchen foil over the fish to make a loose but well-sealed parcel and place it diagonally on the baking tray, so that it fits in the oven. If it’s still too long, bend the tail end upwards. Bake in the centre of the oven for 2½ hours.

After that, remove the salmon from the oven and allow it to completely cool in the kitchen foil before serving. The skin will peel off very easily now the salmon is cooked. Slit the fish down the middle, following the backbone, and ease the fillets away. Serve with the sauce, crisp, dressed salad leaves (such as Cos lettuce), cucumber, and some hot, buttered new potatoes, or potato salad.

Note: For other weights of salmon, the cooking times are as follows: for 2 lb (900 g), 1½ hours; for 3 lbs (1.35 kg), 2 hours; for 5 lb (2.25 kg), 3 hours.

This recipe contains raw eggs.