Preheat the oven to 180°C/160°C fan or barbeque kettle (Weber) to a medium setting. Pat the scotch fillet dry with a paper towel. Mix three-quarters of the butter with the mustard, thyme, and teaspoon of salt and freshly cracked pepper. Rub the scotch fillet with the butter mix and place it in a baking tray, in the oven, or in the indirect-heat area of the BBQ. Cook for 70-90 minutes or until the internal temperature reaches 60°C. Remove from heat, cover loosely with aluminium foil and rest for 20 minutes in a warm place.
Meanwhile, make the gravy by gently simmering the porcini in the beef stock and red wine for 60 minutes or until reduced by two-thirds. Add any pan juices. Melt the remaining butter and add the flour, mixing well to form a roux. Add to the reduced stock, stirring constantly over medium heat for several minutes until thickened. Season to taste.
Slice the beef thinly and served with roast vegetables and the porcini gravy.
