In a cast-iron or other heavy pot, sear the beef on high heat with the oil until well coloured.
Drain excess oil then add the vegetables, garlic, herbs, salt and pepper.
Cover with cold water.
Braise, covered in the oven at 175'C (350'F) for 3 hours. Allow to cool in the stock.
Once allowed to cool, remove the beef brisket and shred into long strands, taking care to remove any fat or gristle.
For the sauce, strain the stock into a saucepan and reduce. If your sauce becomes too thick, add a glass of red wine and cook until you have the right consistency.
Strain with a very fine chinois or sieve.
Trim the stalks and flaps of the mushrooms, then with the gills facing up, place on a shallow oven tray.
Sprinkle with thyme, garlic and oil.
Season, and cook in the oven at 150'C (300'F) for 1 hour, basting frequently.
Remove and discard thyme and garlic then drain excess oil and juices from the mushrooms. Put aside.
The Potato Puree:
Drop potatoes into cold salted water, bring to the boil and cook until soft. (If you are using new potatoes, cook them with the skin to avoid an elastic texture, otherwise, peel the potatoes before cooking.)
Puree them through a rice presser or a hand masher, do not use an electrical machine.
Add cold butter, mix, then adjust the seasoning.
Just before serving, gradually add warm cream or milk until you achieve the right consistency.
There is no standard recipe for the perfect puree, everything depends on the quality and variety of potato you use and on the time of the year.
In a small pan slowly warm the desired quantity of beef, moistened with a touch of sauce.
To present this dish perfectly, use metal or plastic tubing 9 - 10 cm (4 in) in diameter and 6 cm (2 ½ in) high.
Place the tube on each mushroom and cut a perfect round shape, using a small sharp knife.
Put the ring in the centre of a plate and fill with 2 cm (1 in) of potato puree, top with 3 cm (1 ½ in) of beef brisket then remove ring and set mushroom on top.
Drizzle the sauce around the plate and serve.
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