Preheat oven to 400 F with the rack in the middle.

Melt 1 tbsp of butter in a large oven-safe (cast iron skillet or Dutch oven will work) skillet over medium-high heat. Once butter is melted, add pork loin and brown on all sides (about 6 minutes total).

Place skillet in the oven and bake at 400 F for about 13 - 15 minutes, flipping the pork loin over halfway through baking. Bake until center of pork registers 160 F then transfer to a cutting board and let the meat rest for 5 - 10 minutes.

While pork loin is baking, melt remaining 1 tbsp of butter in a skillet over medium-high heat. Add chopped onion and minced garlic to the skillet, and cook for a couple of minutes until onion is translucent and soft. Add sliced mushrooms and stir. Season everything generously with salt and pepper. Let cook for about 5 minutes, stirring occasionally. When mushrooms are tender, add wine and heavy whipping cream. Stir and cook for about 8 minutes, stirring occasionally until sauce thickens. The sauce will thicken more when it cools off. Season the creamy mushroom marsala sauce with salt and pepper to taste.

Slice pork loin in medallions and serve topped with the creamy mushroom marsala sauce.

You can also serve the pork medallions with potatoes and a light salad.