Cut the tenderloins into 5cm thick medallions and flatten slightly. Season the meat with salt and black pepper.
Heat the olive oil and 15g butter in a large frying pan over high heat. When the butter is melted and foaming, add the meat and sear until nicely browned, 2 to 3 min. Flip and cook the other side until the meat is well browned, about another 2 min. Transfer to a plate.
Melt the remaining 15g butter in the pan. Add the onions and a pinch of salt and sauté for about 30 seconds, using a wooden spoon to scrape up any browned bits from the bottom of the pan.
Add the mushrooms and sauté until all of the mushroom liquid has evaporated and the mushrooms are golden, about 3 min.
Season with 1/2 tsp. salt, sprinkle with the flour, and add the Marsala.
Once the Marsala has almost completely evaporated, add the chicken stock and reduce by half, about 3 min.
Stir in the cream and herbs, return the pork and any accumulated juices to the pan, and cook, flipping the pork once, until it’s firm to the touch and still a little pink in the middle (cut into a piece to check), 2 to 4 min.
Serve with mashed potatoes.
