Cut the trimmed tenderloin into six equal pieces. Butterfly each piece by cutting down the middle, stopping before cutting through. Spread open each piece of pork, like opening a book. Place pieces between sheets of plastic wrap. Using the flat side of a meat mallet, pound each piece to an even thickness, about 1/4 inch to 1/2 inch.

Heat one tablespoon of the oil along with one tablespoon of the butter in a sauté pan over medium-high.

Combine the flour, salt, and pepper in a shallow dish. Dredge pork pieces in the flour mixture, shaking off excess. Saute' half the pork in the oil mixture, until the meat is lightly browned, about 2 to 4 minutes per side. Transfer the sautéed pork to a plate; tent with foil to keep warm. Add the remaining tablespoon of oil and butter to the pan.. Sauté the rest of the pork; transfer to the same plate to keep warm.

Add the tablespoon of oil to the same pan. Add the mushrooms; sauté until softened and browned, about 5 to 7 minutes. Add the shallots and garlic to the pan; cook until softened for about 2 to 3 minutes. Stir in the tablespoon of flour; cook for about one minute.

Deglaze the pan with the Marsala wine, scraping up any brown bits. Cook the sauce over low heat until the liquid is reduced by about half. Stir in the half-and-half; simmer for about 5 minutes.

Return the meat and juices from the plate to the pan; heat through. (See Recipe Notes for alternative method of serving.)

Place pork on plates and top with the sauce. Garnish with the minced fresh parsley.