Tip the porcini into a bowl and pour over 250ml boiling water. Leave for 20 mins to soften and plump up, scoop out the mushrooms with a slotted spoon and finely chop, then strain the liquid (leaving behind the last drop as it might contain grit) and set aside.
Set aside about half the wild mushrooms, choosing those with the best shape, then finely chop the remainder. Chop 1 shallot very finely. Heat a large frying pan, add the butter and 1 tbsp olive oil, then cook the chopped shallot and garlic over a medium heat for 3-5 mins until softened. Tip in the chopped porcini and stir around the pan for 2 mins. Add the chopped mushrooms to the frying pan along with the thyme sprigs. Cook for 10 mins 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 a book. 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, evenly thick piece of meat, about the size of an A4 page. Spread the mushroom mixture all over the beef and season really well. Tightly roll up the beef to reform into a log and tie about 8 pieces of string around to secure well.
Heat oven to 200C/180C fan/gas 6. Place a large heavy-based roasting tin over a high heat on the hob (ensure that it will fit in your oven beforehand). Heat some more butter and the other 1 tbsp oil in the tin, then fry the beef, turning every couple of mins, until well browned all over. This will take about 10 mins.
Tip the whole shallots into the tin around the beef and toss them in the oil. Roast for 20-25 mins for rare, 35 mins for medium and 45 mins for well done. If you have a thick piece of meat, it may take a little longer. Ten mins before the end of cooking, stir the whole mushrooms around the pan, coating well in the oil and juices, then leave to roast alongside the meat and shallots. Leave the meat to rest on a board for 10-15 mins and keep the vegetables warm in a serving dish while you make the sauce.
Place the tin back on the hob. Stir in the flour, mixing well into any oil in the pan, then very carefully pour over the brandy, making sure you don’t pour straight from the bottle. Use a small whisk to stir in the brandy, scraping up all the lovely browned bits from the tin. When the brandy has nearly boiled away, pour over the strained mushroom liquid and beef stock. Cook for about 5 mins until the liquid just coats the back of a spoon, then stir in the mustard and creme fraiche. Season, sprinkle with parsley and pour into a gravy boat. Cut the beef into thick slices and serve with the vegetables, gravy and some Parmesan puffs, if you like.