Slice the pork fillet into 5cm thick rounds and beat them flat. In a non-metallic bowl, combine the pork rounds with the olive oil, garlic, lemon juice and seasoning. Marinate in the fridge for at least 30 minutes.
Melt half the butter in a frying pan and cook and stir the mushrooms and onion until soft, but not browning, lower the heat if necessary. Transfer to a shallow ovenproof baking dish.
In a separate pan, melt the remainder of the butter. Remove the pork from the marinade with a slotted spoon and add to the frying pan and cook for 5 minutes, turning over once, until browned. Transfer to the baking dish and lay over the onions and mushrooms. Add the sherry to any juices left in the pan and bring to the boil for 2 minutes. Reduce the heat and add the crème fraîche, stir until heated through. Pour over the pork medallions.
Bake in the preheated oven for 30 minutes. Remove from the oven and serve.
Loved this dish. Made it for our Tuesday night family meal & even my grand daughter aged 9 loved it. Must say that the dish I cooked it in helped which was a pampered chef roaster; kept it moist. Would make this again in a heartbeat. Thanks a million. - 11 Feb 2014