Season the pork fillets with salt and pepper, coat with the mustard and sprinkle with the pistachios. Lay them on a pyrex roasting tray adding the butter, the garlic and the sherry. Tightly cover the tray with tinfoil sealing out the air.
Roast the pork loin at medium heat for approximately 2 hours. At the end, lift off the foil and leave the fillets in the oven until golden brown on both sides.