Cut the loin into medallions, season with salt and pepper and fry in a pan in oil with the clove of garlic. Fry for 2 minutes on both sides, then remove and set aside.
Chop the onion and mushrooms and sauté in the pan. When the mushrooms are about half the size, stir in the cream and crème fraîche and simmer briefly.
Put the medallions in the sauce and let them heat up briefly. Season with salt and serve.