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