Place a saucepan of water over high heat & bring to a boil. Generously season the water with salt & cook the pasta as per the packet instructions.
Place a large pan over high heat. Add in 2 Tbsp (28g) of unsalted butter, allow to melt & heat up. Once hot, add in the sliced scotch fillet, sear for 45 seconds on each side, remove from the pan & place onto a plate to rest. Do this in batches.
Melt the remaining unsalted butter in the same pan, add in the onions & saute for 2 minutes. Add in the garlic & saute for 30 seconds. Add in the swiss brown mushrooms & saute for 5 minutes or until soft & golden brown.
Add in the dijon mustard & brandy to deglaze the pan & cook for 2 minutes, stirring regularly. Add in the beef stock, Worcestershire sauce & flour, mix well & bring to a simmer. Check for seasoning, adjust if necessary, reduce heat to medium & simmer for 4-5 minutes, stirring regularly until the sauce thickens.
Once thick, reduce the heat to low, add in the sour cream & seared beef along with any plate juices, mix well & heat for 1 minute. Remove from the stovetop.
Add in the pappardelle pasta, mix through well to coat, add in some pasta water if need be & serve. Garnish with the flat-leaf parsley. Dig in.