Trim tenderloin of the white silverskin and fat. Cut the tenderloin into medallions that are about 1 ½- 2 inches thick. Season the pork with salt and pepper.
Heat 2 tbsp of oil and 1 tbsp of the butter in a large skillet over medium-high heat Place the pork in the skillet and sear until browned, 3 minutes. Flip and cook the other side until the pork is well browned, about another 2 min. Remove pork from the skillet and place on a plate.
Add the remaining 1 tbsp butter in the skillet. Add the onion and cook for about a minute. Scrape up any browned bits from the bottom of the pan with a wooden spoon. Add the mushrooms and cook about 3 minutes.
Sprinkle with the flour over the mushrooms, and add the Marsala. Allow the Marsala to simmer until almost completely evaporated.
Add the chicken broth and cook until the sauce is reduced by half. Stir in the cream and parsley.
Return the pork to the pan, and cook, flipping the pork once. The pork should still be slightly pink in the center which is about 145° when temped with a meat thermometer.
Season with salt and pepper, if needed. Serve with pasta or potatoes and sprinkling of parsley.