Heat 3 oz butter in a roasting tray.
Season and sprinkle loin with mixed herbs. Quickly brown loin on all sides, before covering with tin foil. Place in oven for 30 min at 190°C, 375°F, gas
mark 5. Remove loin and cool. Use meat juices and vegetable water to make a gravy and thicken with corn flour.
Lightly cook onion in remaining butter. Add mushrooms, season and cook until almost dry. Pour in wine and reduce. Blend in 8 fl oz of gravy; ham and parsley to make a stiff mix. Season and cool.
Roll out pastry in an oblong 1/8" thick (large enough to cover the loin), leaving enough for decoration. Spread the cooled mixture down the centre of pastry - same length as loin. Place loin on top, cover with remaining mixture. Brush edges with egg, draw pastry over loin, sealing neatly. Place on a baking sheet, decorate and brush with egg. Make a hole in top and leave for 20 min.
Place in hot oven for 15 min at 220°C, 425°F, gas mark 7, to set and colour .
Reduce temperature to 180°C, 350°F, gas mark 4, cooking for a further 1 hr 20 min.