In a large pot, melt 1 tablespoon of butter over medium heat. Add chopped onions with a pinch of salt and cook for 30-40 minutes until golden brown and fragrant. Stir in sage during the last 5 minutes and set aside to cool.

Using the same pot, add the remaining tablespoon of butter and turn heat to medium-high. Add fresh spinach and wilt for about 2-3 minutes. Add minced garlic for the last 30 seconds, then combine with the caramelized onions.

Preheat your oven to 450°F (232°C). Butterfly the pork tenderloin by slicing lengthwise, season inside with salt and pepper.

Spread cranberry sauce evenly across the opened tenderloin. Layer with sautéed spinach mixture and caramelized onions, followed by crumbled goat cheese.

Carefully roll the tenderloin back into shape, secure with kitchen twine, and season outside with salt and pepper.

Place stuffed tenderloin in a roasting pan and roast in preheated oven for about 30-35 minutes, until internal temperature reaches 140-145°F (60-63°C).

Remove from oven and let rest for at least 20 minutes. Slice and serve with additional cranberry sauce or favorite sides.