Cut the lamb into 25 mm (1 in) cubes and put them in a bowl. Cover with water acidulated with the juice of 1/2 lemon and leave in the refrigerator to soak for 12 hours or overnight, changing the water 2 or 3 times. This soaking makes the meat white and delicate.

Drain the meat and place it in a deep, flameproof casserole with enough stock to cover, 5 ml (1 tsp) salt and pepper to taste. Bring to the boil, skimming off the scum.

Add the onion, carrots, leek and bouquet garni, cover and simmer very gently for 1 1/2 hours, until tender. Alternatively, put it into an oven at 150°C (300°F) gas 2.

Ten minutes before the lamb is done, simmer the button onions in a little water until just firm, drain and keep warm. Cook the button mushrooms in a little butter until tender, adding a sprinkling of lemon juice to preserve their colour. Keep warm.

When the lamb is tender, melt 25 g (1 oz) butter in a heavy saucepan. Stir in the flour off the heat, then simmer for a minute or two, stirring constantly, to make a pale roux. Gradually add 600 ml (1 pt) stock ladled from the lamb and bring to the boil, stirring. Lower the heat and simmer for 15 minutes, stirring occasionally.

Beat the egg yolks with cream and the juice of 1/2 lemon. Remove the sauce from the heat and add the egg and cream mixture. Stir over a low heat until the sauce thickens.

Drain the lamb, discarding any pieces of bone or fat. Rinse and dry the casserole and return the meat to it.

Strain the white sauce through a fine sieve onto the meat. Carefully fold in the onions and button mushrooms and correct the seasoning, adding a pinch of freshly grated nutmeg and more salt or pepper if necessary. Garnish with the finely chopped parsley and serve at once.