Bring a kettle to the boil then pour 250ml over the porcini mushrooms and leave for 20 minutes.
Scoop out the mushrooms with a slotted spoon and finely chop, then strain the liquid into a bowl and reserve.
Set aside half the wild mushrooms, then finely chop the remainder.
Chop 1 shallot very finely. Heat a large frying pan, add the butter and 1 tablespoon of olive oil, then cook the shallot and garlic for 3-5 minutes until softened.
Tip in the chopped porcini and stir for 2 minutes.
Add in the chopped mushrooms along with the thyme sprigs.
Cook for 10 minutes until lightly browned and any liquid has evaporated. Leave to cool.
Place the beef on a board.
Take a sharp knife and make a slice lengthways along the fillet, about one-third of the way down, so it folds out like.
One side of the beef should now be thicker than the other.
Make another lengthways slice along the thick half of beef so the beef unfolds to a flat, even thick piece of meat.
Spoon and spread the mushroom mixture all over the beef and season generously with salt and pepper.
Lightly roll up the beef to reform into a log and tie about 8 pieces of string around to secure well.
Preheat the oven to 200°C / 400°F / Gas mark 6.
Place a large roasting tin over a high heat on the hob.
Heat some butter and the remaining olive oil in the tin and fry the beef, turning every couple of minutes, until it is well browned all over, about 10 minutes.
Tip the whole shallots into the roasting tin and toss them in the oil.
Roast for 20-25 minutes for rare, 35 minutes for medium and 45 minutes for well done.
Ten minutes before the end of cooking, stir in the whole mushrooms into the pan, coating them well in cooking juices, dot some garlic over the top if you like, then leave to roast alongside the meat and shallots.
Leave the meat to rest on a board for 10-15 minutes and keep the vegetables warm in a serving dish.
Make the sauce by placing the roasting tin back on the hob.
Stir in the flour, mixing well then very carefully pour over the brandy, using a cup, do not pour straight from the bottle.
Whisk to stir in the brandy, scraping up the browned cooking bits from the tin.
When the brandy has nearly boiled away, pour over the strained mushroom liquid and beef stock.
Cook for about 5 minutes until the liquid coats the back of a spoon, then add in the mustard and crème fraiche and stir to combine.
Season with salt and pepper and sprinkle with parsley then pour the gravy into a jug for serving.
Cut the beef into thick slices and serve with the warmed vegetables, gravy.