Preheat your oven to 450°F (232°C). Gather your roasting pan and meat thermometer.

In a skillet over medium-low heat, melt 2 tablespoons of unsalted butter. Add sliced onions and cook for 30–40 minutes until golden brown.

Add fresh spinach to the skillet and sauté for 2–3 minutes until wilted. Mix with the caramelized onions.

Butterfly the pork tenderloin by cutting it lengthwise, open it like a book for filling.

Season the pork tenderloin with salt and pepper. Spread cranberry sauce, then the spinach and onion mixture, followed by crumbled goat cheese.

Roll the pork tenderloin tightly and secure with kitchen twine. Pat the outside with salt and pepper.

Transfer to the roasting pan and roast for 30–35 minutes or until internal temperature reaches 140°F to 145°F (63°C).

Let it rest for about 20 minutes. Then slice the tenderloin into rounds.