Preheat the oven to 220'C.
Finely dice the garlic and chop the flat parsley. Use a sharp knife to make incisions in the beef . Insert the sage into the incisions and then season the meat with salt.
Heat a frying pan and add a little olive oil. Colour both sides of the fillets and then remove from the pan. Allow to rest for at least 5 minutes before placing in the oven and cook for 5 to 8 minutes depending on the thickness of the steak.
Wash all of the mushrooms and trim if necessary. Heat a pan, add olive oil and cook the mushrooms in batches making sure not to overcrowd the pan. Make sure they are a nice golden brown colour. Seaason with salt and pepper each time you add a batch. Finish with the garlic, chopped parsley and then gently stir in the truffle oil.
For the sauteed potatoes: place the potatoes in a pan of cold, salted water. Bring to the boil and cook until the potatoes are tender. Heat a frying pan and add a little olive oil. Add the potatoes to the pan and then add a little butter. Season with salt and pepper and cook until golden brown.
Serve the fillet steaks with the truffled mushrooms and the sauteed potatoes.
«Vary the mushrooms according to taste, season and availability.»