Preheat the oven to 220'C.
Cut the skirt steak into 150g steaks.
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 a little truffle oil if necessary.
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.
For the bearnaise sauce: peel and slice the shallot.
Place the white wine vinegar, white wine, whole white peppercorns, shallot and the tarragon stalks into a saucepan. Place on the heat to reduce by 3/4 and then strain the reduction.
Melt the butter in a separate pan to clarify.
Over a bain marie, whisk the egg yolks and white wine reduction until the mixture triples in volume and is light and fluffy (a sabayon).
Gradually add the melted butter to the sabayon and then whisk continuously to prevent the mixture from splitting.
Chop in the tarragon stalks and season with salt and pepper.
Serve the fillet steaks with the wild mushrooms and the sauteed potatoes. Finish the dish with the bearnaise sauce.
«Vary the mushrooms according to taste, season and availability.»