1. For the duxelles,rehydrate the porcini in 200ml of boiling water. Strain after 20 minutes,reserving the liquid,and chop finely.
2. In a medium pot,add the butter,eschalots and garlic and sweat,without colouring,until softened. Add the porcini,field mushrooms and thyme,season with salt and pepper and cook over medium heat for 15 minutes,stirring frequently. Add the porcini liquid and sherry and reduce until dry. Add the cream and reduce until you have a thick dark paste (it's important that it is quite dry at this point). Chill to stiffen.
3. For the pickled mushrooms,in a small pot bring 300ml of water,the white wine vinegar and salt to a simmer,add the mushrooms and simmer for three minutes.
4. Drain the mushrooms well and toss in a little extra virgin oil,a dash of sherry vinegar and some thyme leaves. Check and adjust the seasoning and set aside at room temperature.
5. For the horseradish cream,combine all ingredients and chill.
6. Preheat the oven to 200C fan-forced or 220C conventional.
7. Season the eye fillet,rub with oil and seal in a pan over medium to high heat. Rotate the fillet to seal well on all sides (including ends);this will take about eight minutes - do this evenly as it is critical to the end result.
8. Place the fillet on a tray and roast for 20 minutes (this will end up being rare to medium rare;if you want it cooked more,do it now as it won't cook much once wrapped in the brioche). Rest the meat on a rack (so the juices can run off) until it reaches room temperature.
9. Smear the chilled duxelles evenly over the fillet,wrap firmly in plastic wrap and chill for 15-20 minutes (don't leave in the fridge longer than this or the meat won't come back to room temperature).
10. Roll the brioche out gently to a rectangle large enough to accommodate the fillet. Unwrap the meat and place on top of the dough. Wrap the dough around the meat,folding the ends in to form a tight parcel - take care to seal the joints properly. Transfer the rolled beef on to a lined baking tray with the seam underneath. Brush well with beaten egg and allow to prove for 20 minutes at room temperature (nowhere too hot). Bake for 25 minutes or until golden brown.
11. Let the beef sit for a couple of minutes before slicing thickly and serving with the pickled mushrooms,horseradish cream and some cooked asparagus.
Drink: A good claret! Or how about a medium-bodied cabernet,maybe something from the Yarra Valley.