This dish relies on its sauce for the delicious effect. A loin of lamb will usually feed 5 people but sometimes only 4 if the eye is rather small. The bigger the better. It is rolled in pastry and can be baked whole or possible even better it can be cut into individual poritons each cooked seperately. It cannot be kept warm for any length of time as the pastry goes soggy.
Do remove any sinew before you roll it up in the pastry, otherwise it will curl and burst through it.
Take out the eye of the loin and also the fillet from underneath the rib cage. Remove the sinew from the loin by working from the tail end and, as if you were skinning a fillet of plaice, side and wiggle the knife up to the neck end where you will also have to remove an extra bit of gristly meat. Remove the gristly meat from the undercut too.
Make a mushroom stuffing (duxelle) by sweating the chopped onions in butter, both to cook it and to boil off as much moisture as possible without browning it. Next add the chopped mushrooms and fry it off with the onion until the mixture is dry. Season with plenty of ground nutmeg and salt and leave it in a sieve to drain and cool.
To make the sauce: Chop the fresh mint leaves and boil down in the vinegar with the butter until the kick had gone but don't singe the vinegar. Add this flavouring to the sabayon and season. Keep warm.
Now roll out the pastry. Season and place the loin unstreched on the front edge with the fillet added to the neck end to even the thickness. Lay the sprigs of fresh lemon thyme along the meat, cover with the mushroom mixture and roll up tightly, seasoning and glazing the pastry with egg wash. Allow to rest and chill. Cut into poritons or leave whole.
Bake for approx 20 minutes in a very hot oven at 230°C/450?F/gas mark 8.
Slice each portion into rounds. Arrange on individual plates with some of the sauce as garnish and serve the rest seperately.