Remove any silver skin from the pork tenderloin. Slice the tenderloin into medallions about 1-inch thick. Season the pieces with salt and freshly ground black pepper. Let them rest for a few minutes.

Heat a large skillet over medium-high heat. Add the butter and allow it to melt. Sear the pork medallions for about 2-3 minutes on each side until they are golden brown.

Remove the pork from the skillet and cover loosely with foil to keep it warm.

In the same pan, reduce the heat to medium. Add the remaining butter and the finely chopped onion (or shallot). Sauté until the onion is soft and translucent.

Add the sliced mushrooms to the pan and cook for about 5 minutes until browned and moisture has been released.

If using garlic, add it to the pan and cook for an additional minute. Deglaze the pan with the beef or vegetable stock, scraping up any flavorful bits.

Stir in the heavy cream and simmer until the sauce thickens slightly, about 3-5 minutes.

Return the pork medallions to the pan and spoon the creamy sauce over them. Let them simmer until cooked through, about 2-3 more minutes.

Sprinkle freshly chopped parsley or thyme over the top and serve immediately.