Heat two tablespoons of the oil in a frying pan. Fry the onion, garlic, carrots and celery for 10-12 minutes, or until softened and golden-brown. (You may need to increase the heat toward the end of the cooking time to encourage the vegetables to brown.) Spoon the cooked vegetables into a casserole dish.
Place the beef fillet onto a chopping board. Cut a pocket in the side of the fillet, leaving a 2cm/1in gap at each end. (Make sure that you only make an incision halfway into the meat and that you don’t cut the meat into two pieces).
Cut the pâté into 1.5cm/½inwide strips, place them inside the pocket and drizzle with the truffle oil, if using. Close the pocket to encase the filling. Season the beef all over with sea salt and plenty of freshly ground black pepper.
Wrap the beef in the bacon rashers and secure with kitchen string. Place a bay leaf between the bacon and string every other rasher. Set aside.
Preheat the oven to 180C/350F/Gas 4.
Heat the frying pan and add the remaining oil and brown the beef fillet in the frying pan for 10-12 minutes, or until the bacon is crisp and golden-brown. Place the beef fillet on top of the vegetables.
Remove all but two tablespoons of fat from the frying pan and stir in the flour. Slowly add the cognac, stirring constantly. Once the mixture is boiling, add the Madeira, followed by the beef stock. Bring the liquid to a simmer then pour immediately around the beef. Cook the beef in the oven for 30-35 minutes for rare beef, or 40 minutes for medium-rare.
Carefully remove the beef onto a chopping board, cover with a piece of foil and two tea towels. Return the casserole to the hob and simmer for 2-3 minutes, or until the liquid thickens slightly. Strain through a fine sieve into a warmed jug. Carve the beef into thick pieces.
Pour some of the sauce into six deep plates. Place the beef on top and garnish with fresh parsley and serve with creamed potatoes and green beans.