Preheat your oven to 450°F (232°C) and line a baking sheet with aluminum foil.

Melt 2 tablespoons of butter in a skillet over medium-low heat. Add sliced onions and a pinch of salt, cooking for 30-40 minutes until caramelized.

In the same skillet, melt 1 tablespoon of butter and sauté fresh spinach until wilted for 2-3 minutes. Add minced garlic for the last 30 seconds.

Butterfly the pork tenderloin by slicing it lengthwise, being careful not to cut all the way through.

Spread a layer of cranberry sauce inside the butterflied pork, followed by sautéed spinach, caramelized onions, and crumbled goat cheese. Roll and secure with kitchen twine.

Season the exterior with salt and pepper. Place seam-side down on the baking sheet and roast for 30-35 minutes until the internal temperature reaches 140-145°F (60-63°C).

Allow to rest for 20 minutes before slicing and serving.