1. In a Dutch oven, heat butter over medium heat until butter is melted. Add onion, mushrooms, red bell pepper, and garlic; cook for 10 minutes, stirring frequently, until tender. Add spinach, rosemary, 1/2 teaspoon salt, and ¼ teaspoon pepper, stirring to combine; cook for 3 minutes. Add cheese, stirring just until combined; set aside.
Preheat oven to 475°.
Place pork loin on a clean surface and cut into one side of meat, leaving a 1/2-inch border. Continue cutting the loin until the loin is able to be rolled out into a rectangle and is about 1/2 inch thick. Place a piece of heavy-duty plastic wrap on top of pork. Using a meat mallet, pound the meat until it reaches an even thickness of about ½ inch. Spread spinach-mushroom filling evenly on prepared pork loin, leaving a 1-inch border. Roll up lengthwise, jellyroll style. Secure with butcher’s twine. Rub outside of stuffed pork loin with 2 tablespoons olive oil and remaining 1/2 teaspoon salt and ¼ teaspoon pepper.
Place, seam side down, in a roasting pan. Bake for 20 minutes. Reduce heat to 325°; loosely cover with aluminum foil. Bake for 45 to 55 minutes, or until a meat thermometer inserted into center of pork loin reaches 155°, or to desired degree of doneness. Let stand for 10 minutes before slicing. Serve with Lemon Herb Sauce.