Caramelize the Onions: Place a cast iron skillet over medium-low heat and melt the 2 tablespoons unsalted butter. Add the 1 medium yellow onion, 1 tablespoon fresh rosemary, and ⅛ teaspoon kosher salt. Cook slowly, stirring every few minutes, for about 20 minutes, until the onions are soft and golden brown. Stir in the 3 tablespoons fresh parsley and 2 cloves garlic and cook 3–4 minutes more. Transfer the mixture to a bowl and set aside.

Preheat the Oven: Preheat the oven to 450°F.

Prepare the Pork Tenderloin: While the onions cook, place the 1 ½ to 2 pounds pork tenderloin on a cutting board. With a sharp knife, make a lengthwise cut down the center, stopping about ¼ inch from the bottom (don’t cut all the way through). Open it up like a book. Cover with plastic wrap and pound evenly to about ¼ inch thickness. Season both sides with half of the 1 teaspoon kosher salt and ½ teaspoon coarse ground black pepper.

Mix the Cheese Filling: In a small bowl, stir together the 2 ounces cream cheese and 2 ounces crumbled feta cheese until smooth and combined.

Layer the Fillings: Spread the ⅓ cup jellied cranberry sauce down the center of the flattened pork, leaving 1 inch clear at both ends. Spread the onion mixture next to the cranberry layer, and then spread the cheese mixture alongside.

Roll and Secure the Pork: Starting from a long side, roll up the pork tightly like a jelly roll. Secure with kitchen twine every 1–2 inches. If you don’t have twine, toothpicks along the seam will work.

Season the Outside: Place the rolled tenderloin seam-side down back in the cast iron skillet (or transfer to a rimmed baking sheet if your skillet isn’t oven-safe). Sprinkle with the remaining salt and pepper.

Roast the Pork: Roast for 28–33 minutes, or until the pork reaches 145°F internally (check with an instant-read thermometer in the thickest part).

Rest Before Slicing: Remove from the oven and let the pork rest for 15–20 minutes before slicing into 1 ½-inch thick rounds. Resting allows juices to redistribute and keeps the meat tender and moist.