Cut off a generous one third of the messiest parts of the eels. Set aside while you make the fish mousse. Put the trimmings you cut away into a liquidiser with the egg whites and reduce to a puree.

Transfer the puree to a bowl set over ice. In another bowl, whip the cream until thick but not stiff, then work it slowly into the eel puree. Add the nutmeg (and seasoning to taste).

Take an earthenware or stoneware terrine and layer into it the eel mousse and the eel fillets. Cut a piece of butter paper to fit the terrine, place it on top, then cover the terrine with a double lid or foil.

Either steam the mousse for 1 1/4 hours or put it into a pan with boiling water to come halfway up the side and bake in a moderate oven until the top is just firm - mark 3-4, 160 °C - 180 °C / 325 - 350 °F / Gas 3 - 4.

Remove the terrine to a cool place. When cold, put in the refrigerator overnight.

Serve with the sauce made as follows: remove enough leaves from the watercress to make a tablespoon when chopped. Liquidise the rest with the minimum amount of water to reduce the watercress to a murky slush. Push it through a sieve, add the cream and whip until thick. Fold in the chopped leaves.