1. Heat a large pot over medium-low heat and add 1 tablespoon of butter. Saute until caramelized, stirring often, approximately 30 minutes. Once caramelized, add remaining butter, sage and spinach. Mix over medium heat until spinach is wilted. Stir in garlic for an additional 30 seconds. Remove from heat and set aside.
2. Preheat the oven to 450°F. Line a roasting pan or baking sheet with aluminum foil and spray it with nonstick spray.
3. Place the pork on a cutting board and with a sharp knife slice it lengthwise (approximately 1/3 in), then cut length wise again allowing the pork to roll open to a flat surface. Season the inside with salt and pepper.
4. Top the pork with the spinach onion mixture. Sprinkle the feta cheese on top.
5. Tightly roll up the tenderloin and tie it together with kitchen twine. Season with the salt and pepper.
6. Place the pork on the roasting pan or baking sheet and roast for 30 to 35 minutes, or until the inside is cooked and reads about 145°F. Let the pork rest for 10 minutes before slicing. Slice and serve with cranberry sauce.