The Duke was not a gourmet, and this is the only dish to which he gave his name. This has been said to be because the finished fillet looks like the leg of a well-polished brown leather boot.
Pre-heat oven to 425° F., gas mark 7.
Trim fat and skin from fillet. Roll out pastry very thinly to give a piece 1¼ times the length of your fillet and wide enough to wrap round it with a good overlap. Spread the fillet all over with slightly softened butter. Lightly spread some pate where the meat is to lie along the middle of the pastry and lay a line of mushrooms, caps downwards, along this. Season lightly. Lay the beef on the mushrooms, spread it all over with the rest of the pate and lay the rest of the mushrooms on it, again seasoning them lightly. Bring up the sides of the pastry to make a neat roll and seal the edges together by moistening with a little milk, making a ridge on top which can be decoratively slashed. Seal the ends tightly. Place on a baking tin on middle shelf of oven for 20 minutes at 425° F., gas mark 7. If browning too quickly, cover with foil.
After 20 minutes, remove the beef, and turn the oven down to 350° F., gas mark 4. Brush the top of the crust with an egg yolk to gild it. Cover lightly with foil and put back for 25 minutes. After a total of 45 minutes you should have really rare beef within the crust. If you like it a little more done, leave it another 10 minutes.
Serve as soon as possible when done (as the beef goes on cooking in the crust if you keep it warm), cutting slices about an inch wide crosswise.
©1975 The Estate of Elizabeth Ayrton