Trim any fat or sinew from beef. Remove the rinds from the bacon and set the rashers aside. Place rinds in a large frying pan and cook over medium heat until some of the fat is rendered. Discard rinds.

Peel and finely chop the onion. Melt the butter in the pan, and saute the onion until transparent. Chop the mushrooms finely and add to the pan with the thyme. Cook over a high heat, stirring frequently, until the mixture is dry.

Mix the arrowroot or cornflour into the milk until smooth and add to the mushrooms. Stir well and

Bacon and chopped mushrooms surround tender beef. cook over a medium heat until the mixture thickens and becomes firm. Season to taste and leave to cool.

Lay bacon rashers on a piece of greaseproof paper so they overlap slightly, making a 'sheet' of bacon. Spread the mushroom mixture evenly over the bacon, then place the beef fillet on top.

Carefully wrap the bacon around the beef, securing it at intervals with cocktail sticks, skewers or string.

Place the beef on a rack in a roasting tin and cook in the oven for 45 minutes for medium-cooked beef, 35 minutes for rare and 55 minutes for well done.

Turn off oven. Remove beef from the tin and keep warm on a serving plate in the oven. Mix a little of the cooking juices into the flour until smooth. Add to roasting tin juices and cook over medium heat, stirring continually until bubbling. Add mustard and stock, a bit at a time, for the sauce.

Simmer for 5 minutes until slightly thickened, season to taste, and strain into sauceboat. Serve beef in thick slices.