2) Transfer the skillet to the oven. Add the mustard, peppercorns or herbs, if desired. Roast until an instant-read thermometer inserted in the centre registers 52C for medium-rare, about 25 minutes.
3) Transfer the roast to a cutting board, tent it very loosely with aluminium foil, and let it rest for 10 to 15 minutes.
4) Slice crosswise, arrange on a platter, and serve with the mushroom ragout.
For the mushroom ragout:
1) Clean the mushrooms by brushing with a tea towel or a brush to remove any loose dirt. Remove the shiitake stems and discard. Trim the dry ends off the cremini and white mushroom stems. Quarter all the mushrooms and put in a bowl.
2) Heat 2 tbsp of the butter in a large skillet over a medium-high heat. Add the mushrooms and spread them out evenly in the pan, increase the heat to high. Let the mushrooms cook undisturbed until they brown, then shake the pan to turn them over. Add the additional butter along the sides of the pan as the mushrooms cook if the pan seems very dry.
3) Continue to cook until the mushrooms are nicely browned, about 5 minutes. Add the shallot and cook until softened, about 2 minutes. Season the mushrooms with the salt and pepper and add the thyme.
4) Pull the pan off the heat and add the Madeira or the wine. Return the pan to the heat and scrape up any of the brown bits that cling to the bottom of the pan with a wooden spoon.
5) Add the double cream and bring to a boil. Remove from the heat and serve.