Begin by heating 1 tablespoon of butter in a wide pot over medium heat. Add sliced onions and cook them gently, stirring regularly, for about 30-40 minutes or until they become golden brown and caramelized.

In the same pot with the caramelized onions, add the remaining tablespoon of butter and heat it until melted. Toss in fresh spinach and minced garlic, stirring until the spinach has wilted down, about 3-4 minutes.

Preheat your oven to 450°F (232°C). Take the pork tenderloin and place it on a cutting board. Butterfly the tenderloin by slicing it lengthwise down the center, ensuring you leave about ½ inch uncut at one end.

Once the pork has been butterflied, season the inside generously with salt and pepper. Spread a layer of cranberry sauce across the opened tenderloin, then distribute the wilted spinach and onion mixture.

Carefully roll the tenderloin from the uncut end toward the filled side, securing it with kitchen twine. Season the exterior with additional salt and pepper.

Place the stuffed pork tenderloin on a greased baking sheet and roast in the preheated oven for about 30-35 minutes, monitoring the internal temperature to reach 140°F to 145°F.

Once roasted, remove the stuffed pork tenderloin from the oven and let it rest for 20 minutes before slicing and serving.