In a large skillet, melt 2 tablespoons of butter over medium heat. Add thinly sliced onions and sauté for 30-40 minutes until caramelized. Add minced garlic and fresh spinach, cooking until the spinach wilts, then set aside to cool.

Butterfly the pork tenderloin lengthwise, opening it like a book. Season with salt and pepper.

Spread cranberry sauce over the pork, add the spinach and onion mixture, then crumble goat cheese on top.

Roll the pork tightly, secure with kitchen twine, and season the outside with salt and pepper.

Preheat oven to 450°F (232°C). Place the rolled tenderloin seam-side down in a roasting pan and roast for 30-35 minutes until the internal temperature reaches 140-145°F.

Let the pork rest for 20 minutes, then slice into thick medallions and serve with extra cranberry sauce.