For the Madeira sauce, heat the butter in a frying pan over a medium heat. When foaming add the shallots and fry for 2-3 minutes, or until just softened. Add the Madeira and thyme, stir well and bring to the boil. Simmer until the liquid has reduced in volume by a third.
Add the jus and simmer gently for 15-20 minutes, or until the liquid has thickened. Add the sherry vinegar and chervil. Season with salt and pepper. Keep warm until ready to serve.
For the creamy mash, cook the potatoes in a saucepan of boiling salted water for 10-15 minutes, or until tender.
Drain the potatoes, then return to the pan and gently warm to remove any excess moisture. Remove from the heat and mash the potatoes with the butter and cream until smooth. Season, to taste, with salt and pepper. Set aside and keep warm.
For the onion rings, whisk the yeast and sparkling water together in a bowl. Add the flour and whisk to a smooth batter. Set aside for 5 minutes.
Fill a deep-fat fryer or a large, deep, heavy-bottomed pan two-thirds full with vegetable oil. Heat the oil to 180C, or until a breadcrumb sizzles and turns golden-brown when dropped into it. (Caution: Hot oil can be dangerous. Do not leave unattended.)
Dip the onion rings in the batter then lower them carefully into the hot oil (you may need to do this in batches). When the onion rings are golden-brown, remove them using a slotted spoon and season with salt and pepper. Drain on kitchen roll.
For the steak, heat a large griddle pan and oil the steak. Place on the griddle pan and cook on each side for 4-5 minutes (this is for well-done steak), depending on how you like your steak cooked. Allow to rest before serving and carving.
For the wild mushrooms, heat a medium frying pan and add the oil, once hot add the mushrooms and cook for 2-3 minutes, and finish with the butter. Season with salt and pepper.
To serve, place a spoonful of creamy mash on the plate. Carve the steak and place alongside the mash. Serve with the mushrooms, sauce and onion rings. Garnish with fresh truffle.