1. Wash and skin the eels and remove the heads and tails.
2. Let them soak a while in salt water, then make the fish stock with the heads and tails.
3. Cut the eels into pieces about 5 cm (2 in) long and put them in the bottom of a pie dish with the chopped onion and parsley.
4. Season with pepper, salt and nutmeg. Add a quarter of the stock to the pie dish.
5. Roll out the pastry, make a good thick lid with a small hole in it and glaze with beaten egg.
6. Cook for 15 minutes at 220°C, 425°F, mark 7, until the pastry begins to puff up and brown, then at 180°C, 350°F, mark 4 for 45 minutes.
7. Cover the pastry with foil just before the pie has finished cooking.
8. Make a sauce by melting the butter and adding the flour and the remaining eel stock, stir until thick, add the lemon juice and pour into the pie before serving hot.