Pat the butterflied pork tenderloin dry and season with salt and pepper on both sides.

In a skillet over medium heat, warm 1 tablespoon olive oil and butter. Add mushrooms and garlic; cook until mushrooms are golden, about 5 minutes. Toss in spinach until wilted. Remove from heat and stir in Parmesan.

Lay prosciutto slices overlapping on a cutting board. Spread the mushroom-spinach mixture evenly. Place the pork tenderloin on top and roll tightly, securing with toothpicks or twine.

Preheat oven to 350°F (175°C). In a hot, oven-safe skillet, heat remaining olive oil. Sear the pork on all sides until golden brown, about 2 minutes per side. Transfer skillet to the oven; roast for 35–40 minutes or until internal temperature reaches 145°F (63°C).

Remove from oven and let rest 10 minutes. Slice into medallions and serve.