Slice pork tenderloin loin filet into 1/2″ thick medallions and season liberally with salt & pepper.
Add olive oil to a large heavy skillet and place over medium-high heat. Cook pork medallions for about 3 minutes per side, until nicely browned on both sides and cooked through. Remove to a plate and set aside; keep warm.
Reduce heat to medium and add butter. When butter has melted add diced onion and mushrooms. Cook until the mushrooms have released all their liquid and the onions are translucent and soft; about 10 minutes.
Add minced garlic, cook for about a minute or two until fragrant.
Add red wine to the skillet and cook for a minute or two, scraping up browned bits in the pan.
Add beef broth, parsley, Worcestershire sauce, and Dijon mustard. Increase heat under skillet as needed to bring mixture to a low boil and allow the gravy to simmer for a minute or two.
Meanwhile, combine milk (or ½ & ½) with cornstarch in a small bowl, mixing until well combined.
Add the cornstarch slurry to the skillet and stir until sauce has thickened.
Return pork medallions to the pan and continue to simmer on low heat until heated through.
Season with additional salt and pepper, to taste.
Serve over a bed of creamy mashed potatoes with mushroom sauce spooned over the top.
Finish with a sprinkling of chopped parsley if desired.
Notes
1 tsp dried parsley can be used in place of fresh.
Keyword family favorite, mushrooms, Pork tenderloin