Mix brandy into melted butter. Smear liberally over fillet. Wrap in foil/gladwrap and refrigerate until required, removing from fridge 1-2 hours prior.
Heat oven to 200C Oil skillet/pan, place unwrapped fillet in pan and surround with mushrooms. Bake 30 minutes. Remove from pan, cover with foil and 2 tea towels 20-30 mins. Reserve mushrooms for sauce.
Meanwhile make sauce by draining juices from pan into saucepan. Add remaining sauce ingredients, bring to boil, add mushrooms. Serve thickly sliced fillet with a lashing of sauce. Seasonal vegetables/green salad and scalloped or roast potatoes ideal accompaniments.